N = int(input())
if N>0 or N<=100:
    A = []
    for i in range(0,N):
      B = int(input())
      if B>0 or B<=1000:
          A.append(B)
 
    sum1 = 1/A[0]
 
    for i in range(0,N-1):
      sum1 = sum1 + 1/A[i+1]
 
    result = 1/sum1
    print(result)