#include <iostream>
using namespace std;
int main(){
	int A, B, C, D;
	cin >> A >> B >> C >> D;
	int ans = 0;
	for (int i = A; i <= B; i++){
		for (int j = C; j <= D; j++){
			if (i != j){
				ans++;
			}
		}
	}
	cout << ans << endl;
}