https://www.acwing.com/problem/content/424/
#include<bits/stdc++.h>
using namespace std;
int l,m;
int tree[100005];
int main(){
cin>>l>>m;
int ans=l+1;
while(m--)
{
int left,right;
cin>>left>>right;
for(int i=left;i<=right;i++)
tree[i]=1;
}
for(int i=0;i<=l;i++)if(tree[i]==1)ans--;
cout<<ans<<endl;
return 0;
}