#include <algorithm>
#include <iostream>
#include <iomanip>
#include <limits.h>
#include <map>
#include <math.h>
#include <numeric>
#include <queue>
#include <set>
#include <sstream>
#include <string>
#include <utility>
#include <vector>
#include <stack>

using namespace std;
#define rep(i, n) for (int i = 0; i < n; i++)
#define rep1(i, n) for (int i = 1; i < n + 1; i++)
#define all(A) A.begin(), A.end()
#define itr(A, l, r) A.begin() + l, A.begin() + r
#define debug(var) cout << #var << " = " << var << endl;
typedef long long ll;

int main(void)
{
    int a, b, c, d;
    cin >> a >> b >> c >> d;
    int ans = (b - a + 1) * (d - c + 1);
    // debug(ans);
    rep1(i, 100)
    {
        if (a <= i && i <= b && c <= i && i <= d)
        {
            ans--;
        }
    }
    cout << ans << endl;
}