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

const int N = 5e3 + 10;

int n;
int a[N << 1];

struct edge {
	int to, flow;
	int nxt;
} e[N << 7];

int head[N << 5], cur[N << 5], id = 0;
void addE(int u, int v, int f) {
	e[id].to = v, e[id].flow = f;
	e[id].nxt = head[u], head[u] = id++;
}
void add(int u, int v, int f) {
	addE(u, v, f);
	addE(v, u, 0);
}

int S, T; 

int dis[N];
queue<int> q; 
bool bfs() {
	memset(dis, -1, sizeof dis);
	q.push(S); dis[S] = 0;
	while (!q.empty()) {
		int u = q.front(); q.pop();
		for (int i = head[u]; ~i; i = e[i].nxt) if (e[i].flow) {
			int v = e[i].to;
			if (dis[v] == -1) {
				dis[v] = dis[u] + 1;
				q.push(v);
			}
		}
	}
	return ~dis[T];
}
int dfs(int u, int a) {
	if (u == T || !a) return a;
	int flow = 0, f;
	for (int i = cur[u]; ~i; i = e[i].nxt) {
		cur[u] = i;
		int v = e[i].to;
		if (dis[v] == dis[u] + 1 && (f = dfs(v, min(a, e[i].flow))) > 0) {
			e[i].flow -= f, e[i ^ 1].flow += f;
			a -= f, flow += f;
			if (!a) break;
		}
	}
	return flow;
}
int Dinic() {
	int ans = 0;
	while (bfs()) {
		memcpy(cur, head, sizeof cur);
		ans += dfs(S, 0x3f3f3f3f);
	}
	return ans;
}

int root, tNode;
int lc[N << 4], rc[N << 4];
void modify(int &p, int sl, int sr, int idx) {
	if (sl == sr) {
		if (!a[sl]) p = sl;
		return;
	}
	++tNode;
	lc[tNode] = lc[p], rc[tNode] = rc[p];
	p = tNode;
	int mid = (sl + sr) >> 1;
	if (idx <= mid) modify(lc[p], sl, mid, idx);
	else modify(rc[p], mid + 1, sr, idx);
	if (lc[p]) add(p, lc[p], 1);
	if (rc[p]) add(p, rc[p], 1);
}
void query(int p, int sl, int sr, int ql, int qr, int u) {
	if (!p) return;
	if (ql <= sl && sr <= qr) return add(u, p, 1);
	int mid = (sl + sr) >> 1;
	if (ql <= mid) query(lc[p], sl, mid, ql, qr, u);
	if (mid < qr) query(rc[p], mid + 1, sr, ql, qr, u);
}

int main() {
	ios::sync_with_stdio(0);
	cin.tie(0);
	
	cin >> n;
	for (int i = 1; i <= n; ++i) {
		int l, r;
		cin >> l >> r;
		a[l] = r;
	}
	
	memset(head, -1, sizeof head); tNode = (n << 1);
	for (int i = 1; i <= (n << 1); ++i) if (a[i]) {
		query(root, 1, (n << 1), i, a[i], i);
		modify(root, 1, (n << 1), a[i]);
	}
	S = tNode + 1, T = tNode + 2;
	for (int i = 1; i <= (n << 1); ++i) {
		if (a[i]) add(S, i, 1);
		else add(i, T, 1);
	}
	
	cout << (n << 1) - Dinic() << endl;

	return 0;
}