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

#define REP(i, n) for(int i = 0; i < (n); i++)
using ll = long long;
using ld = long double;
int main(){
    ll N;
    cin >> N;
    ld ans = 1000.0;
    REP(i, N-1){
        ll A;
        cin >> A;
        ans *= (1000.0-A)/1000.0;
    }
    cout << std::fixed << std::setprecision(15) << 1000.0-ans << endl;
}