#include <bits/stdc++.h>
using namespace std::literals::string_literals;
using i64 = long long;
using std::cout;
using std::endl;
using std::cin;

int main() {
	int a, b, c, d; scanf("%d%d%d%d", &a, &b, &c, &d);
	
	int ans = 0;
	for(int A = a; A <= b; A++) for(int B = c; B <= d; B++) ans += (A != B);
	printf("%d\n", ans);
	return 0;
}