前缀和和差分
#include<iostream>
using namespace std;
int solve()
{
int L,M;
cin>>L>>M;
int ans=0;
int* arr=new int[L+1];
while(M--)
{
int x,y;
cin>>x>>y;
arr[x]+=1;
arr[y+1]-=1;
}
for(int i=0;i<=L;i++)
{
if(i>0){
arr[i]=arr[i-1]+arr[i];
}
if(!arr[i]) ans++;
}
return ans;
}
int main()
{
cout<<solve()<<endl;
return 0;
}