#include <bits/stdc++.h>
using namespace std;

int main() {
    long n;
    cin >> n;
    
    vector<long> canisters(n);
    for (long i = 0; i < n; ++i) {
        cin >> canisters[i];
    }

    vector<long> c(n+1,0);
    for (long i = 0; i < n; ++i) {
        c[canisters[i]]++;
    }

    long z = 0;
    for (long i = 0; i <= n; ++i) {
        while(c[i]-- > 0) {
            canisters[z++]= i;
        }
    }
    
    float v = 1.0;
    for (long i = 0; i < n; ++i) {
        if (canisters[i] > i+1) {
            cout << "impossible" << endl;
            return 0;
        } else {
            v = v < float(canisters[i])/(i+1) ? v : float(canisters[i])/(i+1);
        }
    }
    
    cout << v << endl;
    return 0;
}