import numpy as np
 
 
def main():
    n = int(input())
    nums = np.array(list(map(int, input().split())))
    n_total = nums.prod()
    n_denominator = sum([n_total/num for num in nums])
    ans = n_total / n_denominator
    print(ans)
  
  
if __name__ == '__main__':
    main()