枚举 · 例8扩展-校门外的树：hard(牛客)

枚举 · 例8扩展-校门外的树：hard

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

typedef pair<int, int> PII;
#define x first 
#define y second

signed main(){
    int l, n; cin >> l >> n;
    vector<PII> p(n);
    for(int i = 0; i < n; ++i){
        cin >> p[i].x >> p[i].y;
    }
    sort(p.begin(), p.end());
    int res = 0, ed = 0;
    for(int i = 0; i < n; ++i){
        if(ed < p[i].x){res += p[i].x - ed - (ed == 0 ? 0 : 1); ed = p[i].y;}
        else ed = max(ed, p[i].y);
    }
    res += l - ed;
    cout << res << endl;
    
    return 0;
}