#include <iostream>
int main(){
	int l, n,vu[100][2],u,v;
	scanf_s("%d %d", &l, &n);
	for (int i = 0; i < n; i++)
	{
		scanf_s("%d %d", &u,&v);
		vu[i][0] = u;
		vu[i][1] = v;
	}
	int* ll = new int[l];
	for (int i = 0; i < l; i++)
	{
		ll[i] = 1;
	}
	for (int i = 0; i < n; i++)
	{
		for (int j = vu[i][0]-1; j <= vu[i][1]-1; j++)
		{
			ll[j] = 0;
		}
	}
	int z = 1;
	for (int i = 0; i < l; i++)
	{
		z += ll[i];
	}
	std::cout << z;
	return 0;
}