//A
#include <iostream>
#include <cstdio>
#include <set>
#include <map>
#include <algorithm>
#include <vector>
#include <queue>
#include <stack>
#include <cstring>
#include <cassert>
#include <cmath>

#define forn(i,n) for(int i = 0; i < (int)(n);i++)
#define forall(i,c) for(typeof((c).begin()) i = (c).begin(); i != (c).end();i++)
#define forsn(i,s,n) for(int i = (int)(s); i < (int)(n);i++)
#define all(v) (v).begin(),(v).end()
#define isIn(i,c) ((c).find(i) != (c).end())
#define SQR(x) ((x)*(x))

typedef long long tint;

using namespace std;

const int MAXL = 1000100;
int pmp[MAXL];

void preMp(string& x){
    int i=0, j = pmp[0] = -1;
    while(i<(int)x.size()){
        while(j>-1 && x[i] != x[j]) j = pmp[j];
        pmp[++i] = ++j;
    }
}

string s;
vector<int> prefs;

int main(){
    #ifdef JUAMPI
        freopen("PASSWORD.in","r",stdin);
        freopen("PASSWORD.out","w",stderr);
    #endif

    getline(cin,s);
    preMp(s);

    int n = s.size();

    int k = pmp[n];
    /*forn(i,n) cout << pmp[i+1] << " ";
    cout << endl;
    */
    for(int i = pmp[n]; i >= 0; i--){
        if(k == i){
            prefs.push_back(i);
            k = pmp[i];
        }
    }
    prefs.pop_back();
    /*
    forn(i,prefs.size()){
        cout << "ONE: " << endl;
        forn(j, prefs[i])
            cout << s[j] << " ";

        cout << endl;
    }
    */
    sort(all(prefs));

    int l = -1, r = prefs.size();
    while(r - l > 1){
        int m = (l+r)/2;

        bool ok = false;
        forsn(i,1,n-1) if(pmp[i+1] == prefs[m]) ok = true;
        if(ok) l = m; else r = m;
    }
//  cout << l << " " << r << endl;

    if(l == -1) puts("Just a legend"); else{
        forn(i,prefs[l]) putchar(s[i]);
        putchar('\n');
    }
    return 0;
}