problem
solution
codes
#include<iostream>
using namespace std;
int a[100010], ans;
int main(){
int l, m; cin>>l>>m;
for(int i = 0; i <= l; i++)a[i] = 1;
for(int i = 0; i < m; i++){
int x, y; cin>>x>>y;
for(int j = x; j <= y; j++)a[j] = 0;
}
for(int i = 0; i <= l; i++)if(a[i])ans++;
cout<<ans;
return 0;
}