#include <iostream>
#include <string>
#include <set>
#include <map>
#include <vector>
#include <queue>
#include <algorithm>
#include <bitset>
#include <cstdlib>
#include <cmath>
#define INF 100000000
#define ll long long int
using namespace std;




int main(){
int A,B,C,D; cin>>A>>B>>C>>D;
int pre = (B-A+1) * (D-C+1);
int cnt=0;
vector <int> box; box.resize(200,0);
for(int i=A;i<=B;i++) box[i]++;
for(int i=C;i<=D;i++) box[i]++;
for(int i=0;i<=120;i++){
  if(box[i]==2) cnt++;
}
cout<<pre-cnt<<endl;
  return 0;
}