#include<bits/stdc++.h>
using namespace std;
typedef long long int ll;
#define IOS ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
typedef tree<int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update>order_set;
typedef pair<int, int>pr;
#define all(i) i.begin() , i.end()
#define ft first
#define sn second
#define pb push_back
#define totalone(mask) __builtin_popcount(mask)
#define chkbit(mask,bit) (mask&(1LL << bit))
// debug section start
void __print(int x) {cerr << x;}
void __print(long x) {cerr << x;}
void __print(long long x) {cerr << x;}
void __print(unsigned x) {cerr << x;}
void __print(unsigned long x) {cerr << x;}
void __print(unsigned long long x) {cerr << x;}
void __print(float x) {cerr << x;}
void __print(double x) {cerr << x;}
void __print(long double x) {cerr << x;}
void __print(char x) {cerr << '\'' << x << '\'';}
void __print(const char *x) {cerr << '\"' << x << '\"';}
void __print(const string &x) {cerr << '\"' << x << '\"';}
void __print(bool x) {cerr << (x ? "true" : "false");}
template<typename T, typename V>
void __print(const pair<T, V> &x) {cerr << '{'; __print(x.first); cerr << ','; __print(x.second); cerr << '}';}
template<typename T>
void __print(const T &x) {int f = 0; cerr << '{'; for (auto &i : x) cerr << (f++ ? "," : ""), __print(i); cerr << "}";}
void _print() {cerr << "]\n";}
template <typename T, typename... V>
void _print(T t, V... v) {__print(t); if (sizeof...(v)) cerr << ", "; _print(v...);}
#ifndef ONLINE_JUDGE
#define debug(x...) cerr << "[" << #x << "] = ["; _print(x)
#else
#define debug(x...)
#endif
// debug section closed
#define en "\n"
#define sum(n) ((1LL*(n)*(n+1))/ 2LL)
#define sqr(n) (1LL*(n)*(n))
#define vag(a,b) ((a + b - 1)/b)
#define MAXN 200010
#define inf 1e16+5
const int mod = 1e9 + 7;
struct edge {
ll to, cs;
};
ll n, m;
vector<edge>g[MAXN];
void dijkstra()
{
vector<vector<ll>>dis(n + 1, vector<ll>(2, inf));
priority_queue<vector<ll>, vector<vector<ll>>, greater<vector<ll>>>pq;
pq.push({0, 1, 1});
dis[1][1] = 0;
while (!pq.empty())
{
auto t = pq.top(); pq.pop();
ll nd = t[1];
ll dist = t[0];
ll st = t[2];
if (dist > dis[nd][st]) continue;
for (auto i : g[nd])
{
if (st == 1)
{
if (dis[i.to][1] > dist + i.cs) {
dis[i.to][1] = dist + i.cs;
pq.push({dis[i.to][1], i.to, 1});
}
if (dis[i.to][0] > dist + i.cs / 2) {
dis[i.to][0] = dist + i.cs / 2;
pq.push({dis[i.to][0], i.to, 0});
}
}
else {
if (dis[i.to][0] > dist + i.cs) {
dis[i.to][0] = dist + i.cs;
pq.push({dis[i.to][0], i.to, 0});
}
}
}
}
cout << min(dis[n][0], dis[n][1]) << en;
}
void solve()
{
cin >> n >> m;
for (ll i = 1; i <= m; i++)
{
edge e;
ll nd;
cin >> nd >> e.to >> e.cs;
g[nd].pb(e);
}
dijkstra();
}
int main()
{
IOS;
ll t;
t = 1;
// cin >> t;
int c = 0;
while ( t-- )
{
// cout<<"Case "<<++c<<": ";
solve();
}
return 0;
}