#include <bits/stdc++.h>
using namespace std;
const int maxn = 100005;
double p[maxn], dp[maxn];
int n;
int main() {
  int i;
  while (scanf("%d", &n) != EOF) {
    double ans = 0;
    for (i = 1; i <= n; i++) {
      scanf("%lf", &p[i]);
      ans += p[i];
    }
    dp[0] = 0;
    for (i = 1; i <= n; i++) {
      dp[i] = dp[i - 1] * p[i] + p[i];
      ans += (dp[i] - p[i]) * 2;
    }
    printf("%.7f\n", ans);
  }
  return 0;
}
