r1, r2, b1, b2 = map(int, input().split(' '))

ans = (r2 - r1 + 1) * (b2 - b1 + 1)
for i in range(r1, r2 + 1):
    for j in range(b1, b2 + 1):
        if i == j:
            ans -= 1

print(ans)