import math
 
def area_circle(r):
    return math.pi * r * r
 
def area_rectangle(a, b):
    return a * b
 
def area_triangle(a, h):
    return a * h / 2
 
def area_trapezoid(a, b, h):
    return (a + b) * h / 2
 
 
choice = int(input())
 
if choice == 1:
    r = float(input())
    print(area_circle(r))
 
elif choice == 2:
    a, b = map(float, input().split())
    print(area_rectangle(a, b))
 
elif choice == 3:
    a, h = map(float, input().split())
    print(area_triangle(a, h))
 
elif choice == 4:
    a, b, h = map(float, input().split())
    print(area_trapezoid(a, b, h))
 

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