Submission #73389185

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Source Code Expand

#include<bits/stdc++.h>
#define ll long long
#define ull unsigned long long
#define lp(a, b, c) for(int a=(b);a<=(c);a++)
#define lq(a, b, c) for(int a=(b);a>=(c);a--)
#define tsk(t) int t;cin>>t;while(t--)
#define _if(a,b,c) ((b)?(a):(c))
#define endl '\n'
using namespace std;
const int N = 2e5+3; struct node {
    int L, R;
} a[N];
int main() {
    ios::sync_with_stdio(false);
    cin.tie(0), cout.tie(0);
    int n, m; cin>>n>>m; lp(i,1,m) cin>>a[i].L>>a[i].R;
    sort(a+1, a+m+1, [](node a, node b) {
        if(a.L == b.L) return a.R > b.R;
        return a.L < b.L;
    });
    int curR = 1, maxR = 1, ans = 0;
    int i=1; while(i<=m && curR <= n) {
        if(a[i].L > curR) { cout<<-1<<endl; return 0; }
        while(i<=m && a[i].L <= curR) {
            maxR = max(maxR, a[i].R);
            i++;
        }
        curR = maxR+1; ans++;
    }
    if(curR <= n) { cout<<-1<<endl; return 0; }
    cout<<ans<<endl;
    return 0;
}

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