Link to this code: https://cses.fi/paste/eb1576bc1900b84776a439/
#include<bits/stdc++.h>
#define ll long long

using namespace std;

const ll p = 101, mod = 1e9+7;
string s;

int main(){
	cin.tie(nullptr)->sync_with_stdio(0);
//	freopen("in_all.txt", "r", stdin);
// 	freopen("out_all.txt", "w", stdout);
	cin >> s;
	int n=s.size(), l=1, r=n, idx=-1;
	while(l<r){
		int m = (l+r+1)>>1;
		bool ok=false;
		ll hs=0, fr=1;
		map<ll, bool> mp;
		for(int i=0;i<m;i++) hs=hs*p%mod+s[i]-'a', fr=fr*p%mod;
		hs%=mod;
		mp[hs]=1;
		for(int i=m;i<n;i++){
			hs=hs*p%mod+s[i]-'a';
			hs-=fr*(s[i-m]-'a')%mod;
			hs+=mod, hs%=mod;
			if(mp.find(hs)!=mp.end()){
				ok=true;
				idx=i;
				break;
			}
			mp[hs]=1;
		}
		if(ok) l=m;
		else r=m-1;
	}
	if(idx==-1) cout << -1;
	else for(int i=idx-l+1;i<=idx;i++) cout << s[i];
	
	return 0;
}