Shared code

/*      ###########################
        #  Author : Pranay Garg   #
        #  College : SGSITS       #
        ###########################
*/
# include   <bits/stdc++.h>
# include   <ext/pb_ds/assoc_container.hpp>
# define    ll                  long long int
# define    ull                 unsigned long long int
# define    double              long double
# define    sp(x,y)             fixed << setprecision(y) << x
# define    ironman             ios_base::sync_with_stdio(false);cin.tie(NULL);
# define    MOD                 1000000007
# define    MODT                998244353
# define    endl                '\n'
# define    clr(x,a)            memset(x,a,sizeof(x))
# define    ar                  array
# define    INF                 1000000000000000000ll + 239
# define    PI                  3.14159265358979323846
# define    fi                  first
# define    se                  second
# define    vi                  std::vector<ll>
# define    pb                  push_back
# define    pi                  pair<ll,ll>
# define    all(a)              a.begin(), a.end()
# define    allr(a)             a.rbegin(), a.rend()
# define    alla(a,n)           a,a+n
# define    debug1(x)           cout<<#x<<" = "<<x<<endl;
# define    debug2(x,y)         cout<<#x<<" = "<<x<<" "<<#y<<" = "<<y<<endl;
# define    debug3(x,y,z)       cout<<#x<<" = "<<x<<" "<<#y<<" = "<<y<<" "<<#z<<" = "<<z<<endl;
# define    debug4(x,y,z,w)     cout<<#x<<" = "<<x<<" "<<#y<<" = "<<y<<" "<<#z<<" = "<<z<<" "<<#w <<" = "<<w<<endl;
# define    debug5(x,y,z,w,v)   cout<<#x<<" = "<<x<<" "<<#y<<" = "<<y<<" "<<#z<<" = "<<z<<" "<<#w <<" = "<<w<<" "<<#v <<" = "<<v<<endl;
# define    debug6(x,y,z,w,a,b) cout<<#x<<" = "<<x<<" "<<#y<<" = "<<y<<" "<<#z<<" = "<<z<<" "<<#w <<" = "<<w<<" "<<#a<<" = "<<a<<" "<<#b<<" = "<<b<<endl;

# define    max(...)({ \
    ll nos[] = { __VA_ARGS__ }; \
    ll n = sizeof(nos)/sizeof(ll); \
    *std::max_element(&nos[0],&nos[n]); \
})

# define    min(...)({ \
    ll nos[] = { __VA_ARGS__ }; \
    ll n = sizeof(nos)/sizeof(ll); \
    *std::min_element(&nos[0],&nos[n]); \
})

using namespace std;
using namespace __gnu_pbds;

typedef tree<int, null_type, less<int>, rb_tree_tag,
        tree_order_statistics_node_update> indexed_set;

template<typename... T>
void read(T&... args)
{
    ((cin >> args), ...);
}

template<typename... T>
void write(T... args)
{
    ((cout << args << " "), ...);
}

void fastio()
{
    ironman
#ifndef ONLINE_JUDGE
    freopen("test_input.txt", "r", stdin);
    freopen("output.txt", "w", stdout);
    freopen("error.txt", "w", stderr);
#endif
}

template<class T>
void printVector(const std::vector<T> arr)
{
    for (auto i : arr)
        cout << i << " ";
    cout << endl;
}
template<class T>
void printArray(T arr[], ll n)
{
    for (ll i = 0; i < n; i++)
        cout << arr[i] << " ";
    cout << endl;
}

template<class T>
void printUGraph(vector<T> arr[], ll n)
{
    for(ll i=0;i<n;i++)
    {
        cout << i << " : ";
        for(auto j : arr[i])
            cout << j << " ";
        cout << endl;
    }
}

template<class T>
void printWGraph(vector<T> arr[], ll n)
{
    for(ll i=0;i<n;i++)
    {
        cout << i << " : ";
        for(auto j : arr[i])
            cout << j.fi << "," << j.se << " ";
        cout << endl;
    }
}
//const int p1 = 13331,m1 = 1e9+9,p2 = 7919, m2 = 1e9+9;
//const int p1 = 97266353,m1 = 972663749,p2 = 97336357, m2 = 1000000007;
//const int p1 = 31 ,m1 = 1e9+7;

//grid cells
//D U R L
//ll dx[] = {1, -1, 0, 0};
//ll dy[] = {0, 0, 1, -1};
// bool isSafe(ll x, ll y, ll n, ll m)
// {
//     return (x >= 0 && x < n && y >= 0 && y < m);
// }


//---------------TEMPLATE ABOVE--------------//
const int maxn = 2e5+5;
vector<ar<ll,2>> graph[maxn];
//deque<ll> ans;
vi ans;
ll n,m,c=0;
vi deg,vis;
void dfs(ll node)
{
//    debug1(node);
    for(auto i : graph[node])
    {
        if(!vis[i[1]])
        {
            c++;
            vis[i[1]] = 1;
            dfs(i[0]);
        }
    }
//    ans.push_front(node);
    ans.pb(node);
}
void solve()
{
    read(n,m);
    deg.assign(n,0);
    for(ll i=0;i<m;i++)
    {
        ll u,v;
        read(u,v);
        u--;v--;
        deg[u]++;
        deg[v]++;
        graph[u].pb({v,i});
        graph[v].pb({u,i});
    }
    ll odd = 0,s=0;
    for(ll i=0;i<n;i++)
    {
        odd+=((deg[i]%2)==1);
    }
    if(odd!=0)
    {
        cout << "IMPOSSIBLE" << endl;
        return;
    }
    vis.assign(m,0);
    dfs(0);
    if(c!=m)
    {
        cout << "IMPOSSIBLE" << endl;
        return;
    }
    reverse(all(ans));
    for(auto i : ans)
        cout << i+1 << " ";
    cout << endl;
}
int main()
{
    ironman
    fastio();
    ll t = 1;
    ll cases = t;
    while (t--)
    {
        // cout << "Case #" << cases-t << ": ";
        solve();
    }
}