#include <bits/stdc++.h>
using namespace std;
const int N = 1000000;
int z[N];
void computeZ(char s[]) {
  int n = strlen(s), L = 0, R = 0;
  for (int i = 1; i < n; i++) {
    if (i > R) {
      L = R = i;
      while (R < n && s[R - L] == s[R]) R++;
      z[i] = R - L;
      R--;
    } else {
      int k = i - L;
      if (z[k] < R - i + 1)
        z[i] = z[k];
      else {
        L = i;
        while (R < n && s[R - L] == s[R]) R++;
        z[i] = R - L;
        R--;
      }
    }
  }
}
char s[N + 1];
int suffix[N + 1], size;
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cin >> s;
  computeZ(s);
  int n = strlen(s), mx = 0;
  int size = 0;
  for (int i = 1; i < n; ++i)
    if (z[i] == n - i) suffix[z[i]]++;
  for (int i = 1; i < n; ++i) {
    if (suffix[i])
      suffix[i] = i;
    else
      suffix[i] = suffix[i - 1];
  }
  for (int i = 1; i < n; ++i) {
    int len = z[i] - (z[i] == n - i);
    mx = max(mx, suffix[len]);
  }
  if (mx == 0)
    cout << "Just a legend" << endl;
  else {
    s[mx] = '\0';
    cout << s << endl;
  }
}
