#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using P = pair<int,int>;
#define rep(i,n) for(int i=0;i<n;i++)

const ll mod = ll(1e9) + 7;
const int INF = 1<<29;

int main(){
    int a,b,c,d;
    cin >> 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++;
        }
    }
    cout << ans << endl;
    return 0;
}