#include <bits/stdc++.h>
#define int long long 
using namespace std;


signed 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;


}