#include <bits/stdc++.h>
using namespace std;
int ns, k, n, m;
char s[1000010], t[1000010], p[1000010];
vector<int> vet;
int fail[1000010];
void buildFail(char *p) {
  int j = fail[0] = -1;
  for (int i = 1; i <= m; i++) {
    while (j >= 0 && p[j] != p[i - 1]) j = fail[j];
    fail[i] = ++j;
  }
}
int kmp(char *p, char *t) {
  buildFail(p);
  for (int i = 0, k = 0; i < n; i++) {
    while (k >= 0 && p[k] != t[i]) k = fail[k];
    if (++k >= m) {
      return 1;
      k = fail[k];
    }
  }
  return 0;
}
int test(int k) {
  int i, j;
  m = 0;
  for (i = 0, j = ns - k; i < k; i++, j++) {
    if (s[i] != s[j]) break;
    p[m++] = s[i];
  }
  if (i < k) return 0;
  p[m] = '\0';
  return kmp(p, t);
}
int main() {
  int lo, hi, mid;
  scanf(" %s", s);
  ns = strlen(s);
  vet.clear();
  m = ns;
  buildFail(s);
  k = ns;
  while (fail[k] > 0) {
    vet.push_back(fail[k]);
    k = fail[k];
  }
  sort(vet.begin(), vet.end());
  for (int i = 1; i < ns - 1; i++) t[i - 1] = s[i];
  t[n - 2] = '\0';
  n = ns - 2;
  lo = 0;
  hi = vet.size() - 1;
  k = 0;
  while (lo <= hi) {
    mid = (lo + hi) / 2;
    if (test(vet[mid])) {
      k = vet[mid];
      lo = mid + 1;
    } else
      hi = mid - 1;
  }
  if (!k)
    printf("Just a legend\n");
  else {
    for (int i = 0; i < k; i++) printf("%c", s[i]);
    printf("\n");
  }
  return 0;
}
