#include<stdio.h>
#include<iostream>
#include<algorithm>
#include<vector>
#include<string>
#include<utility>
#include<map>
#include<set>
#include<queue>
#include<stack>
#include<functional>
#include<math.h>
#include<random>
#include <bitset>
#include <cassert>
#include <complex>
using namespace std;
#define N (1000000000+7)
//#define N 998244353
#define INF 1e16
typedef long long ll;
typedef pair<int,int> P;
const int inf = 1e8;



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)continue;
            ans++;
        }
    }
    cout<<ans<<endl;
}