#include<iostream>
#include<string>
#include<algorithm>
#include<cctype>
#include<set>
#include<bitset>
#include<math.h>
#include<map>
#include<queue>
#include<iomanip>
using namespace std;

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