#include <iostream>
using namespace std;
const int N(10003);
bool t[N]={false};
int main()
{
	int l,m,ans(0);
	cin>>l>>m;
	int x,y;
	for(int i=1;i<=m;i++)
	{
		cin>>x>>y;
		for(int j=x;j<=y;j++)
			t[j]=true;
	}
	for(int i=0;i<=l;i++)
		if(!t[i])
			ans++;
	cout<<ans<<endl;
	return 0;
}
/**************************************************************
	Problem: 2253
	User: admin
	Language: C++
	Result: Accepted
	Time:48 ms
	Memory:2084 kb
****************************************************************/