#include <bits/stdc++.h>
#define rep(i,n) for(int i = 0; i < (n); ++i)
#define srep(i,s,t) for (int i = s; i < t; ++i)
#define drep(i,n) for(int i = (n)-1; i >= 0; --i)
using namespace std;
typedef long long int ll;
typedef pair<int,int> P;
#define yn {puts("Yes");}else{puts("No");}
#define MAX_N 200005

int main() {
    int a, b, c, d;
    cin >> a >> b >> c >> d;

    int ans = (b - a + 1) * (d - c  + 1);

    srep(i,a,b+1){
        srep(j,c,d+1){
            if(i == j)ans--;
        }
    }

    cout << ans << endl;
    return 0;
}