#include <iostream>
#include <algorithm>
#include <cstring>
#include <vector>

using namespace std;
const int N = 1010;

int n;
struct Segment {
	int x, l, r, d;
	bool operator < (const Segment &other) const {
		return x < other.x;
	}	
}seg[N << 1];

struct Node {
	int l, r;
	int cnt, len;
} tr[N << 3];

vector<int> ys;

int find(int x) {
	return lower_bound(ys.begin(), ys.end(), x) - ys.begin();
}

void pushup(int u){
    if (tr[u].cnt) tr[u].len = ys[tr[u].r + 1] - ys[tr[u].l];
    else if (tr[u].l != tr[u].r)
        tr[u].len = tr[u << 1].len + tr[u << 1 | 1].len;
    else tr[u].len = 0;
}

void build(int u, int l, int r) {
	tr[u] = {l, r, 0, 0};
	if (l == r) return ;

	int mid = (l + r) >> 1;
	build(u << 1, l, mid), build(u << 1 | 1, mid + 1, r);
}

void modify(int u, int l, int r, int d) {
	if (tr[u].l >= l && tr[u].r <= r) {
		tr[u].cnt += d;
		pushup(u);
		return ;
	}
	int mid = (tr[u].l + tr[u].r) >> 1;
	if (l <= mid) modify(u << 1, l, r, d);
	else if (r > mid) modify(u << 1 | 1, l, r, d);
	pushup(u);
}

int main() {
	cin >> n;

	int k = 0;
	for (int i = 0; i < n; i ++ ) {
		int x1, y1, x2, y2;
		cin >> x1 >> y1 >> x2 >> y2;
		seg[k ++ ] = {x1, y2, y1, 1};
		seg[k ++ ] = {x2, y2, y1, -1};
		ys.push_back(y1), ys.push_back(y2);
	}

	sort(ys.begin(), ys.end());
	ys.erase(unique(ys.begin(), ys.end()), ys.end());

	build(1, 0, ys.size() - 2);

	sort(seg, seg + n * 2);

	int res = 0;
	for (int i = 0; i < n * 2; i ++ ) {
		if (i) res += tr[1].len * (seg[i].x - seg[i - 1].x);
		modify(1, find(seg[i].l), find(seg[i].r) - 1, seg[i].d);
	}

	cout << res << endl;

	return 0;
}