#include <iostream>
#include<string>
#include<cmath>
#include<algorithm>
#include<cctype>
#include<queue>
#include<deque>
#include<regex>
#include<stack>
#include<stdio.h>
#include<vector>
#include<set>
#include<map>
#include<iomanip>

#define rep(i, n) for(int i=0;i<n;i++)

typedef int long long ll;
using namespace std;
int dx[4] = {1, 0, -1, 0};
int dy[4] = {0, 1, 0, -1};
const ll MOD = 1e9 + 7;
int m[101];
int main() {
    int a,b,c,d;
    cin>>a>>b>>c>>d;
    int an=b-a+1;
    int bn=d-c+1;
   for(int i=a;i<=b;i++){m[i]++;}
    for(int i=c;i<=d;i++){m[i]++;}
    int j=0;
    rep(i,101){
        if(m[i]==2){j++;}
    }

    int ans=an*bn;
   ans-=j;
    cout<<ans<<endl;





    return 0;
}