# -*- coding: utf-8 -*-

# Luis Yoichi Morales January 2019



a, b, c = list(map(int, input().split()))

print(a*b /2)



# -*- coding: utf-8 -*-

# Luis Yoichi Morales January 2019



ab, bc, ca = list(map(int, input().split()))

print(ab*bc/2)

# -*- coding: utf-8 -*-
# Luis Yoichi Morales January 2019
a, b, c = list(map(int, input().split()))
print(a * b / 2)

# -*- coding: utf-8 -*-
# Luis Yoichi Morales January 2019
ab, bc, ca = list(map(int, input().split()))
print(ab * bc / 2)

-a, b, c = list(map(int, input().split())) -print(a * b / 2) +ab, bc, ca = list(map(int, input().split())) +print(ab * bc / 2)