A, B, C, D = map(int, input().split())
overlap = max(B - C + 1 , 0)
ans = (B - A + 1 - overlap)*(D - C + 1 - overlap)
ans += overlap * (D - C + 1 - 1) + (B - A + 1 - 1) * overlap
ans -= overlap * (overlap - 1) if overlap > 0 else 0
print(ans)