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


int main(){
    int n;  cin >> n;
    vector<int> a(n - 1);
    for(auto& x : a){
        cin >> x;
    }

    double p = 1.0, m = 1000.0;
    for(auto& x : a){
        p *= (m - x) / m;
    }
    double ans = m * (1.0 - p);
    cout << setprecision(16) << ans << endl;

    return 0;
}