#include <bits/stdc++.h>
using namespace std;
const long long MOD = 1e9 + 7;
const long long B = 727;
const long long MAXn = 1e6 + 100;
string s;
long long n, hsh[MAXn], pw[MAXn];
vector<long long> v;
long long hs(long long l, long long r) {
  if (l == 0) return hsh[r];
  return (hsh[r] - ((hsh[l - 1] * pw[r - l + 1]) % MOD) + MOD) % MOD;
}
bool is_ok(long long x) {
  if (hs(0, x) != hs(n - x - 1, n - 1)) return 0;
  for (long long i = x + 1; i < n - 1; ++i)
    if (hs(0, x) == hs(i - x, i)) return 1;
  return 0;
}
long long bs(long long l, long long r) {
  if (l == r) return r;
  long long mid = (l + r + 1) / 2;
  if (is_ok(v[mid]))
    bs(mid, r);
  else
    bs(l, mid - 1);
}
signed main() {
  cin >> s;
  n = (long long)s.size();
  pw[0] = 1;
  for (long long i = 1; i < MAXn; ++i) {
    pw[i] = (pw[i - 1] * B) % MOD;
  }
  hsh[0] = s[0];
  for (long long i = 1; i < n; ++i) {
    hsh[i] = ((hsh[i - 1] * B) % MOD + (s[i])) % MOD;
  }
  for (long long i = 0; i < n - 1; ++i)
    if (hs(0, i) == hs(n - i - 1, n - 1)) v.push_back(i);
  if ((long long)v.size() == 0) {
    return cout << "Just a legend\n", 0;
  }
  long long nn = (long long)v.size();
  long long point = bs(0, nn - 1);
  if (is_ok(v[point])) {
    for (long long i = 0; i <= v[point]; ++i) cout << s[i];
    cout << endl;
  } else {
    return cout << "Just a legend\n", 0;
  }
  return 0;
}
