#include <bits/stdc++.h>
using namespace std;

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

	int ans = 0;
	for( int a = A; a <= B; a++ ) {
		for( int b = C; b <= D; b++ ) {
			if( a == b ) continue;
			ans++;
		}
	}

	cout << ans << endl;
}