import sys
input == sys.stdin.readline

def main():
    #1:初期化
    N = int(input())
    p = list(map(float, input().split()))
    dp = [[0]*(N+1) for _ in range(N+1)]

    #2:初期値
    dp[0][0]=1

    #3:dp実行部
    for i in range(1,N+1):
        for j in range(i+1):
            #print(i,j)
            if j>0:
                dp[i][j] = dp[i-1][j]*(1-p[i-1])+dp[i-1][j-1]*p[i-1]
            else:
                dp[i][j] = dp[i-1][j]*(1-p[i-1])

    #4:出力
    #print(*dp,sep="\n")
    print(sum(dp[N][(N+1)//2:]))

if __name__ == "__main__":
    main()