#include<iostream>
using namespace std;
short int num[10010];
int main()
{
	int sum,m[110],x,i,j,t=0;
	cin>>sum>>x;
	for(j=0;j<=sum;++j)
		num[j]=j;
	for(i=1;i+1<=2*x;i+=2){
		cin>>m[i]>>m[i+1];
		for(j=0;j<=sum;++j){
			if(num[j]!=-1){
				if(num[j]>=m[i] && num[j]<=m[i+1])
					num[j]=-1;
				}
			}
		}
		for(j=0;j<=sum;++j){
			if(num[j]!=-1)
				++t;
	}
	cout<<t;
	return 0;
 }