#include<bits/stdc++.h>

using namespace std;
using ll = long long;


int main()
{
    int cnt = 0;
    int a, b, c, d; cin >> a >> b >> c >> d;
    for(int i = a; i <= b; i++) {
        for(int j = c; j <= d; j++) {
            if(i == j)continue;
            cnt++;
        }
    }
    cout << cnt << endl;
}