#include <iostream>
#include <string>
#include <vector>
#include <map>
#include <cmath>
#include <algorithm>
#include <functional>
#include <stack>
#include <numeric>
using ll = long long;
using namespace std;
const int MOD = (int)1e9 + 7;
const ll INF = (ll)1e18 + 7;
#define int long long
#define vint vector<int>
#define pint pair<int,int>
#define pb(a) push_back(a)
#define all(v) v.begin(),v.end()
#define dbg(x) cerr << #x << " = " << (x) << endl
#define yn(b) cout << ((b) ? "Yes" : "No") << endl
#define YN(b) cout << ((b) ? "YES" : "NO") << endl

int dx[4] = {0, 1, 0, -1};
int dy[4] = {1, 0, -1, 0};

signed main()
{
	cin.tie(0);
	ios::sync_with_stdio(false);

	int A, B, C, D;
	cin >> A >> B >> C >> D;

	vint X(120, 0);
	int cnt = 0;

	for (int i = A; i <= B; i++) {
		X[i]++;
	}

	for (int i = C; i <= D; i++) {
		X[i]++;
	}

	for (int i = 0; i < 110; i++) {
		if(X[i]==2){
			cnt++;
		}
	}

	cout << (B - A + 1) * (D - C + 1) - cnt << endl;

	return 0;
}