D

#include <iostream>
#include <iomanip>
#include <cstdlib>
#include <ctime>
#include <cstdio>
#include <algorithm>
#include <cmath>
#include <vector>
#include <set>
#include <stack>
#include <map>
#include <queue>
#include <string>
#include <memory.h>
#include <iterator>
#define ll long long
#define ull unsigned long long
#define ul unsigned long
#define ld long double
#define mem(A,X) memset(A,X,sizeof(A))
#define memo(A) memset(A,0,sizeof(A))
#define forn(I,B) for (I=1;I<=(B);I++)
#define repp(I,B) for (int I=0;I<(B);I++)
#define fornn(I,B) for (int I=1;I<=(B);I++)
#define rep(I,B) for (I=0;I<(B);I++)
#define sz(X) ((int)(X.size()))
#define lg(X) ((int)(X.length()))
#define cl(X) (X).clear()
#define labs(X) (((X)>0)?(X):(-(X)))
#define INF 1111111111
#define ropen(X) freopen(X,"r",stdin)
#define wopen(X) freopen(X,"w",stdout)
#define bg(X) (X.begin())
#define en(X) (X.end())
#define vi vector<int>
#define pii pair<int,int>
#define pull pair<ull,ull>
#define pb push_back
#define mp make_pair
#define all(X) (X).begin(),(X).end()
#define N 1111111
#define P1 31
#define P2 29


using namespace std;

string s;
int n,p[N];
pull h[N];
ull p1[N],p2[N];

//    http://codeforces.ru/contest/127/problem/D

void init(){
    cin>>s;
    n=s.length();
    p1[0]=p2[0]=1;
    fornn(i,n-1){
        p1[i]=p1[i-1]*P1;
        p2[i]=p2[i-1]*P2;
    }
}

void kmp(){
    p[0]=0;
    for (int i=1;i<n;i++){
        int j=p[i-1];
        while (j>0 && s[i]!=s[j]) j=p[j-1];
        if (s[i]==s[j]) ++j;
        p[i]=j;
    }
}

void hash_calc(){
        h[0].first=h[0].second=s[0]-'a'+1;
    fornn(i,n-1){
        h[i].first=h[i-1].first+(s[i]-'a'+1)*p1[i];
        h[i].second=h[i-1].second+(s[i]-'a'+1)*p2[i];
    }
}

bool f(int m){
    if (m==0) return 1;
    ull h1=h[m-1].first,h2=h[m-1].second;
    h1*=p1[n-m];
    h2*=p2[n-m];
    if ((h1!=h[n-1].first-h[n-m-1].first) || (h2!=h[n-1].second-h[n-m-1].second)) return 0;
    for (int i=m-1;i<=n-m-1;i++) if (p[i]>=m) return 1;
    return 0;
}

void sol(){
    int l=0,r=p[n-1];
    int m=0;
    while (l<r){
        m=(l+r)/2;
        if (f(m)) l=m+1;
        else r=m;
    }
    if (f(p[n-1]))++r;
    if (r<2){
        cout<<"Just a legend"<<endl;
        return;
    }
    repp(i,r-1) cout<<s[i];
    cout<<endl;
}

int main(){
    ios_base::sync_with_stdio(0);
    init();
    kmp();
    hash_calc();
    sol();
}