#include <malloc.h>
#include <stdio.h>

int main(int argc, char **argv) {
  int n; scanf("%d", &n);
  int *v = (int*) malloc(sizeof(int) * n);
  int *c = (int*) malloc(sizeof(int) * (n+1));
  for (int i = n; i --> 0;) {
    int x; scanf("%d", &x); c[x]++;
  }
  for (int i = 0, j = 0; i <= n; i++) {
    while (c[i]--) {
      v[j++] = i;
    }
  }

  float res = 1.0;
  for (int i = 0; i < n; i++) {
    float candidate = v[i] / (float) (i+1);
    if (candidate < res) {
      res = candidate;
    }
    if (v[i] > i+1) {
      puts("impossible");
      goto finish;
    }
  }

  printf("%.8f\n", res);

  finish:
    free(c);
    free(v);

  return 0;
}