#include<iostream>
#include<vector>
#include<algorithm>
#include<string>
#include<unordered_set>
using namespace std;

int main(){
    int A,B,C,D;
    cin >> A >> B >> C >> D;

    if(B > D) swap(A,C),swap(B,D);
    int ans = (B-A+1)*(D-C+1);
    if(B-C >= 0) ans -= (B-max(A,C)+1);

    cout << ans << endl;
}