代码比较易读，暂不作解释

#include<bits/stdc++.h>

using namespace std;

#define l first
#define r second

typedef pair<int, int> PII;
vector <PII> segs;
int n, l, r, cnt;

int main()
{
    cin >> n;
    while(n --)
    {
        cin >> l >> r;
        segs.push_back({l, r});
    }

    sort(segs.begin(), segs.end());

    l = segs[0].l, r = segs[0].r, cnt = 1;
    for(int i = 1; i < segs.size(); i ++)
        if(segs[i].l <= r) r = min(r, segs[i].r);
        else cnt ++, l = segs[i].l, r = segs[i].r;

    cout << cnt;
}