Pyramid number

def solution(x,y):
    ID = 1

    if x==0 or y==0:
        raise ValueError

    else:
        # looking for the first number of every row
        for n in range(2,y+1):
            ID += 1 + (n-2)

        # sums out all the difference
        for n in range(2, x+1):
            ID += (2 + (n-2)) + (y-1)
        print ID

solution(0,0)