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

signed main () {

    int a, b, c, d;
    scanf("%d %d %d %d", &a, &b, &c, &d);
    int ans = 0;
    for ( int i = a; i <= b; i++ ) {
        for ( int j = c; j <= d; j++ ) {
            if ( i != j ) {
                ans++;
            }
        }
    }
    printf("%d\n", ans);

    return 0;
}