//Normal

#define _GLIBCXX_DEBUG
#define ll long long
#include <bits/stdc++.h>
using namespace std;
using Graph = vector<vector<int>>;

void print(auto a){
  cout << a;
}

void prints(auto a){
  cout << a << " ";
}

void prints(){
  cout << " ";
}

void printl(auto a){
  cout << a << endl;
}

void printl(){
  cout << endl;
}

void fix(int n){
  cout << fixed << setprecision(n);
}

int 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++;
    }
  }
  
  printl(ans);
  
  return 0;
}