#include <iostream>
#include <cstdio>
using namespace std;
int l,m;
bool road[10005];
int main()
{
	cin>>l>>m;
	for (int i=1;i<=m;i++)
	{
		int begin,end;
		cin>>begin>>end;
		for (int i=begin;i<=end;i++)
			road[i]=1;
	}
	int ans=0;
	for (int i=1;i<=l;i++)
		if (road[i]==0)
			ans++;
	cout<<ans;
	return 0;
}