#include <bits/stdc++.h>
using namespace std;
using i64 = long long;
#define rep(i,s,e) for(int (i) = (s);(i) <= (e);(i)++)
#define all(x) x.begin(),x.end()

int main() {
  i64 a,b,c,d;
  cin >> a >> b >> c >> d;
  i64 ans = 0;
  rep(i,a,b) {
    ans += d - c + 1;
    if(c <= i && i <= d) {
      ans--;
    }
  }
  cout << ans << endl;
}