Untitled

type = input()
if type == "3": 
    ##треугольник
    a = int(input())
    b = int(input())
    c = int(input())
    p = (a + b + c)/2
    print((p * (p - a)*(p - b)*(p - c)) ** 0.5)
elif type == "4": 
    ##квадрат
    a = int(input())
    b = int(input())
    print(a * b)
elif type == "inf": 
    ##круг
    r = int(input())
    print(3.141592653589793 * r ** 2)