#include<cstdio>
double dp[3000][3000];
int main(){
  int n;
  scanf("%d",&n);
  dp[0][0]=1;
  for(int i=1;i<=n;i++){
    double p;
    scanf("%lf",&p);
    dp[i][0]=dp[i-1][0]*(1-p);
    for(int j=1;j<i;j++)dp[i][j]=dp[i-1][j-1]*p+dp[i-1][j]*(1-p);
    dp[i][i]=dp[i-1][i-1]*p;
  }
  double ans=0;
  for(int i=n;i>n/2;i--)ans+=dp[n][i];
  printf("%.10lf",ans);
  return 0;
}