import sys


def input(): return sys.stdin.readline().rstrip()


def main():
    R, C = map(int, input().split())
    if R == C:
        if R % 2 == 0:
            print((R//2)**2 - 1)
        else:
            print(((R-1)//2)**2 + (R-1) // 2)
    else:
        print((R*C+1) // 2 - 1)


if __name__ == '__main__':
    main()