#define _USE_MATH_DEFINES
#include  <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
#include <string>
#include<math.h>
#include<iomanip>
#include<stdio.h>
#include <stdlib.h>
#include<stdio.h>
#include <queue>
#include<map>
#include <fstream>
#include <sstream>
#include<set>
#include<stack>
#include<time.h>
#include <sys/types.h>
#include <tuple>
#include <numeric>


typedef long long int ll;
//#include<bits/stdc++.h>


using namespace std;

int64_t modpow(int64_t x, int64_t n, int64_t MOD) {
    
    x = x % MOD;
    if (n == 0) return 1;  //再帰の終了条件

    else if (n % 2 == 1) {
        return (x * modpow(x, n - 1,MOD)) % MOD;  //nが奇数ならnを1ずらす
    }
    else return modpow((x * x) % MOD, n / 2,MOD) % MOD;  //nが偶数ならnが半分になる
}





int main()
{

    int Q;
    Q = 1;
    //cin >> Q;

    while (Q--) {
        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++;
            }
        }
        cout << ans << endl;
    }
    return 0;
}