#include <bits/stdc++.h>
using namespace std;
const long long int INF = 1e18;
inline long long int in() {
  long long int p = 0;
  register char ch = 0;
  while (ch < '0' or ch > '9') {
    ch = getchar();
  }
  while (ch >= '0' and ch <= '9') {
    p = (p << 1) + (p << 3) + ch - '0';
    ch = getchar();
  }
  return p;
}
long long int par[1000001];
set<long long int> s;
void kmp(string st) {
  long long int k;
  memset(par, 0, sizeof(par));
  par[0] = k = 0;
  for (long long int i = 1; i < st.size(); ++i) {
    while (k > 0 && st[k] != st[i]) k = par[k - 1];
    if (st[k] == st[i]) k++;
    par[i] = k;
    if (i != (st.size() - 1)) s.insert(k);
  }
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  string str;
  cin >> str;
  kmp(str);
  long long int ans = par[str.size() - 1];
  if (ans != 0 && s.count(ans) == 1)
    cout << str.substr(0, ans) << endl;
  else {
    ans = par[ans - 1];
    if (ans != 0 && s.count(ans) == 1)
      cout << str.substr(0, ans) << endl;
    else
      cout << "Just a legend" << endl;
  }
  return 0;
}
