#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
#define int long long
#define mk make_pair
#define pb push_back
#define pf push_front
typedef pair<int, int> pii;
#define INF (1 << 30)
#define INFL (1ll << 60ll);
#define mod 1000000007
#define se second
#define fi first

int a, b, c, d;

signed main()
{
    cin >> a >> b >> c >> d;
    int ans = 0;
    for(int i = a; i <= b; i++){
        if(c <= i && i <= d){
            ans += d - c;
        }
        else {
            ans += d - c + 1;
        }
    }
    cout << ans << endl;



    return 0;
}

/*



*/