#include<bits/stdc++.h>
#define rep(i,a,b) for(int i=int(a);i<int(b);++i)
using namespace std;
typedef long long ll;
int INF = (1LL << 30) - 1;
int MOD = 1e9+7;
main(){
    ll A,B,C,D;
    cin >> A >> B >> C >> D;
    ll ans = 0;
    rep(i,A,B+1){
        if(i < C || i > D)ans += D - C + 1;
        else ans += D - C;
    }
    cout << ans << endl;
}