#include <iostream>
using namespace std;
int m[100900],q[100090],w[100090];
int main(){
int a,s,j=0;
cin>>a>>s;
for(int n=0;n<2*s;n+=2){int d,f;
	cin>>d>>f;
	w[n]=d;w[n+1 ]=f;
	
}
	for(int n=0;n<=a;n++){
		m[n]=n;q[n]=m[n]+1;
	}
for(int n=0;n<2*s;n+=2){
for(int i=0;i<=a;i++){int p=i; 
	if(w[n]==m[p]){
		for(m[p];m[p]<=w[n+1];p++)q[p]=0;
	}
 	
}}
for(int n=0;n<=a;n++){
		if(q[n]!=0){
		j++;}
	}cout<<j;
	return 0; 
}