def G(M, V, T, A):
    sum = 0.0
    for i in range(V+1):
        sum += q(i, M, T, A)
    return sum


def q(n, M, T, A):
    if n <= 0:
        return 1

    sum = 0.0
    for i in range(1, M):
        sum += A[i]*T[i]*q(n - T[i], M, T, A)

    return sum/n


def main():
    M = int(input('Podaj M: '))
    V = int(input('Podaj V: '))
    T = []
    A = []
    for index in range(M):
        T.insert(index, int(input(f'Podaj T{index}: ')))
        A.insert(index, float(input(f'Podaj A{index}: ')))

    print(q(V, M, T, A) / G(M, V, T, A))


if __name__ == "__main__":
    main()