#include <bits/stdc++.h>
using namespace std;
#define all(v) (v).begin(),(v).end()

int main() {
	int n;
	while (cin >> n) {
		if (n == 0) {
			cout << 0 << '\n';
			continue;
		}
		vector<int> x1(n), x2(n), y1(n), y2(n);
		vector<int> allx, ally;
		for (int i = 0; i < n; i++) {
			cin >> x1[i] >> y1[i] >> x2[i] >> y2[i];
			allx.push_back(x1[i]);
			allx.push_back(x2[i]);
			ally.push_back(y1[i]);
			ally.push_back(y2[i]);
		}
		sort(all(allx));
		sort(all(ally));
		allx.resize(unique(all(allx)) - allx.begin());
		ally.resize(unique(all(ally)) - ally.begin());
		int cntx = allx.size();
		int cnty = ally.size();
		vector<vector<int>> d(cntx + 1, vector<int>(cnty + 1));
		for (int i = 0; i < n; i++) {
			int i1 = lower_bound(all(allx), x1[i]) - allx.begin();
			int i2 = lower_bound(all(allx), x2[i]) - allx.begin();
			int j1 = lower_bound(all(ally), y1[i]) - ally.begin();
			int j2 = lower_bound(all(ally), y2[i]) - ally.begin();
			d[i1][j1]++;
			d[i1][j2]--;
			d[i2][j1]--;
			d[i2][j2]++;
		}
		for (int i = 0; i <= cntx; i++) {
			for (int j = 0; j <= cnty; j++) {
				if (i > 0) d[i][j] += d[i - 1][j];
				if (j > 0) d[i][j] += d[i][j - 1];
				if (i > 0 && j > 0) d[i][j] -= d[i - 1][j - 1];
			}
		}
		long long ans = 0;
		for (int i = 0; i < cntx; i++) {
			for (int j = 0; j < cnty; j++) {
				if (d[i][j] > 0) {
					long long dx = allx[i + 1] - allx[i];
					long long dy = ally[j + 1] - ally[j];
					ans += dx * dy;
				}
			}
		}
		cout << ans << '\n';
	}
}