#include <bits/stdc++.h>
using namespace std;
int main(){
    int a, b, c, d;
    cin >> a >> b >> c >> d;
    int64_t ans = 0;
    for(int i = a; i <= b; ++i){
        if(i < c || i > d)  ans += d - c + 1;
        else    ans += d - c;
    }
    cout << ans << endl;
}