#include <iostream>
using namespace std;
int main() {
int s1=0, s2=0, a, b, c, d, i;
cin >> a >> b >> c >> d;
for (i = a; i <= b; i++) {
s1 += i;
}
for (i = c; i <= d; i++) {
s2 += i;
cout << s1 - s2;
return 0;
