#include "bits/stdc++.h"
#define in std::cin
#define out std::cout
#define rep(i,N) for(LL i=0;i<N;++i)
typedef long long int LL;

int main()
{
	LL A, B, C, D;
	in >> A >> B >> C >> D;

	LL ans = (B - A + 1)*(D - C + 1);
	LL a = std::max(A, C), b = std::min(B, D);
	if (a <= b) ans -= b - a + 1;

	out << ans << std::endl;
}