#include <iostream>
using namespace std; 
int main(){
	int l,m;
	int start,end; 
	cin>>l>>m;
	int a[l+1];
	for(int i=0;i<=l;i++){
		a[i]=1;
	}
	for(int i=0;i<m;i++){
		cin>>start>>end;
		//cout<<"yes"<<endl;
		for(int j=start;j<=end;j++){
			a[j]=0; 
			//cout<<j<<" set 0"<<endl;
		}
	}
	int js=0;
	for(int i=0;i<=l;i++){
		//cout<<i<<" "<<a[i]<<endl;
		if(a[i]==1) {
			js++;
		}
	}
	cout<<js<<endl;
	return 0;
}