#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <queue>
#include <cmath>
#include <climits>
#include <iomanip>
#include <set>
#include <map>
using namespace std;
typedef long long ll;
int main(){
    ll a,b,c,d;
    cin >> a >> b >> c >> d;
    ll res = 0;
    for(int i = a;i <= b;i++){
        for(int j = c;j <= d;j++){
            if(i != j)res++;
        }
    }
    cout << res << endl;
}