from fractions import Fraction
from decimal import Decimal
n = int(input())
a = list(map(int, input().split()))
ans = Fraction(1000, 1)
for i in a:
    ans *= 1000 - i
    ans /= 1000
ans = 1000 - ans
s = Decimal(str(ans.numerator))
s /= Decimal(str(ans.denominator))
print(s)