#include <bits/stdc++.h>
using namespace std;
int n;
long double p[3005];
long double dp[3005][3005];

main() {
    ios_base::sync_with_stdio(false);
    cin.tie(0); cout.tie(0);
    freopen("TEST.inp", "r", stdin);
    freopen("TEST.out", "w", stdout);
    cin >> n;
    for (int i = 1; i <= n; i++) cin >> p[i];

    dp[1][0] = 1 - p[1];
    dp[1][1] = p[1];
    for (int i = 2; i <= n; i++) {
        for (int j = 0; j <= i; j++) {
            dp[i][j] += dp[i - 1][j - 1] * p[i] + dp[i - 1][j] * (1 - p[i]);
        }
    }
    long double ans = 0;
    for (int i = n / 2 + 1; i <= n; i++) ans += dp[n][i];

    cout << fixed << setprecision(9) << ans << '\n';

    return 0;
}