#include<iomanip>
#include<cmath>
#include<cstring>
using namespace std;
const int N=1e5+10;
const int INF=0x3f3f3f3f;
int l,m,x,y,ans;
bool a[N];
int main()
{
	cin>>l>>m;
	while(m--)
	{
		cin>>x>>y;
		for(int i=x;i<=y;i++)
		a[i]=1;
	}
	for(int i=0;i<=l;i++)
	{
		if(!a[i])
		ans++;
	}
	cout<<ans;
    return 0;
}