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import sys

def solve(n, t, p):
    dp = [0.0] * (n + 1)
    dp[0] = 1.0  

    for prob in p:
        for j in range(n, 0, -1):
            dp[j] = dp[j] * (1 - prob) + dp[j - 1] * prob  

    result = sum(dp[t:])  
    print(f"{result:.20f}")  

if __name__ == "__main__":
    n, t = map(int, sys.stdin.readline().split())
    p = [float(sys.stdin.readline()) for _ in range(n)]
    solve(n, t, p)