A, B, C, D = map(int, input().split())

# [l, r], [src, dst] 両方に含まれる整数の数
def koushiten(l, r, src, dst):
    return max(0, min(r, dst) - max(l, src) + 1)

ans = (B - A + 1)*(D - C + 1) - koushiten(A, B, C, D)
print(ans)