#include <cstdio>
#include <iostream>
#include <string>
#include <vector>
#include <sstream>
#include <map>
#include <set>
#include <queue>
#include <algorithm>
#include <cmath>
#include <cstring>
#include <typeinfo>
#include <numeric>
#include <functional>
#include <unordered_map>
#include <bitset>
#include <stack>


using namespace std;
using ll = long long;
using ull = unsigned long long;

const ll INF = 1e16;
const ll MOD = 1e9 + 7;

#define REP(i, n) for(ll i = 0; i < n; i++)





int main() {
    ll a, b, c, d;
    cin >> a >> b >> c >> d;
    ll ans = 0;
    for(ll i = a; i <= b; i++){
        for(ll j = c; j <= d; j++){
            if(i != j) ans++;
        }
    }
    cout << ans << endl;
}