#include "bits/stdc++.h"
using namespace std;
#define Would
#define you
const long long INF = 1e18;
const long long MOD = 1e9 + 7;
const double pi = acos(-1);
const int SIZE = 1 << 17;
int dx[] = { 1,0,-1,0 }, dy[] = { 0,1,0,-1 }, alp[30];
long long fac[100005], finv[100005], inv[100005];

int main() {
	int a, b, c, d, ans = 0;
	cin >> a >> b >> c >> d;
	for (int i = a; i <= b; ++i) {
		for (int j = c; j <= d; ++j) {
			if (i != j) { ++ans; }
		}
	}
	cout << ans << endl;
}