题解

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

int main()
{
    int a,b;
    cin>>a>>b;
    int d[a+1];
    for(int i = 0;i<a+1;i++)
        d[i]=i;
    for(int i = 0;i<b;i++){
        int x,y;
        cin>>x>>y;
        for(int j = 0;j<a+1;j++){
            if(x<=d[j]&&d[j]<=y){
                d[j]=-1;
            }
        }
    }
    int cnt = 0;
    for(int i = 0;i<a+1;i++)
        if(d[i]!=-1)
            cnt++;
    cout<<cnt;
}