import sys

def debug(x, table):
    for name, val in table.items():
        if x is val:
            print('DEBUG:{} -> {}'.format(name, val), file=sys.stderr)
            return None

def solve():
    R, C = map(int, input().split())

    if R == C:
        ans = (R**2 + 3) // 4
    else:
        ans = (R * C + 1) // 2

    ans -= 1

    print(ans)

if __name__ == '__main__':
    solve()