def add(x, y):
    return x + y

def substact(x, y):
    return y - x

def devide(x, y):
    if y == 0:
        return "division by zero"
    else:
        return x / y
def add(x, y):
    return x + y

def substact(x, y):
    return y - x

def devide(x, y):
    if y == 0:
        return "division by zero"
    else:
        return x / y