#include <bits/stdc++.h>
using namespace std;

typedef long long ll;
const int INF = 0x3f3f3f3f;
const int mod = 1e9 + 7;
const int N = 2e5 + 10;
struct node {
	ll x, yh, yl;
	int flag;
}E[N * 4];
struct edge {
	ll l, r, flag, len;
}tree[N * 4];
ll rk[N * 4], val[N * 4];
int n, cnt = 1;
ll ans, max_y;

void pushup(int id) {
    if (tree[id].l == max_y && tree[id].r == max_y) {
        return;
    }
    if (tree[id].flag) {
        tree[id].len = val[tree[id].r + 1] - val[tree[id].l];
    } else {
		tree[id].len = tree[id << 1].len + tree[id << 1 | 1].len;
    }
}

void build(int id, int l, int r) {
	tree[id].l = l;
	tree[id].r = r;
	tree[id].flag = tree[id].len = 0;
	if (l == r) {
		return;
	}
	int mid = (l + r) >> 1;
	build(id << 1, l, mid);
	build(id << 1 | 1, mid + 1, r);
}

void update(int id, ll l, ll r, ll val) {
	if (l <= tree[id].l && tree[id].r <= r) {
		tree[id].flag += val;
		pushup(id);
		return;
	}
	int mid = (tree[id].l + tree[id].r) >> 1;
	if (l <= mid) {
		update(id << 1, l, mid, val); 
	}
	if (r > mid) {
		update(id << 1 | 1, mid + 1, r, val);
	}
	pushup(id);
}

bool cmp(const node& x, const node& y) {
	if (x.x != y.x) {
		return x.x < y.x; 
	}
	return x.flag > y.flag;
}

int main() {
    ios::sync_with_stdio(0);
    cin.tie(0), cout.tie(0);	
    cin >> n;
    for (int i = 0; i < n; i++) {
		ll x, y, xx, yy;
		cin >> x >> y >> xx >> yy;
		E[cnt].x = x;
		E[cnt].yh = yy;
		E[cnt].yl = y;
		E[cnt].flag = 1;
		rk[cnt++] = y;
		E[cnt].x = xx;
		E[cnt].yh = yy;
		E[cnt].yl = y;
		E[cnt].flag = -1;
		rk[cnt++] = yy;		
	}
	sort(rk + 1, rk + cnt + 1);
	int len = unique(rk + 1, rk + cnt + 1) - (rk + 1);
	for (int i = 1; i <= cnt; i++) {
		ll pos1 = lower_bound(rk + 1, rk + len + 1, E[i].yh) - (rk + 1);
		ll pos2 = lower_bound(rk + 1, rk + len + 1, E[i].yl) - (rk + 1);
		val[pos1] = E[i].yh;
		val[pos2] = E[i].yl;
		E[i].yh = pos1;
		E[i].yl = pos2;
		max_y = max(max_y, pos1);
	}
	sort(E + 1, E + cnt + 1, cmp);
	build(1, 1, cnt);
	for (int i = 1; i < cnt; i++) {
		update(1, E[i].yl, E[i].yh - 1, E[i].flag);
		ans += tree[1].len * (E[i + 1].x - E[i].x);
	}
	cout << ans;
    return 0;
}

样例：

输入

2
100 100 200 200
150 150 250 255

输出

18000

输出 $20750$，求调。