def ans(x1, x2, y1, y2):
    x = (x2 - x1 + 1) * (y2 - y1 + 1) - relu((min(x2, y2) - max(x1, y1) + 1))
    return x

def relu(v):
    return max(0,v)

if __name__ == '__main__':
    a, b, c, d = map(int, input().split())
    print(ans(a, b, c, d))