#include <bits/stdc++.h> using namespace std; typedef long long ll; typedef pair<int,int> pii; #define pb push_back #define mp make_pair #define rep(i,n) for(int i=0;i<(n);++i) const int mod=1000000007; int main(){ cin.tie(0); ios::sync_with_stdio(false); int a,b,c,d;cin >> a >> b >> c >> d; if(d<a || c>b){ cout << (b-a+a)*(d-c+1) << endl; } else if(c<a && d>b){ cout << (b-a+1)*(d-c+1)-(b-a+1) << endl; } else if(c<a && d>=a){ cout << (b-a+1)*(d-c+1)-(d-a+1) << endl; } else if(c<=b && d>b){ cout << (b-a+1)*(d-c+1)-(b-c+1) << endl; } else{ cout << (b-a+1)*(d-c+1)-(d-c+1) << endl; } }