#include<bits/stdc++.h>
#define ll long long
#define N 2000001
using namespace std;
ll n=0;
ll ptime,l=0,flag[N],start[N],pend[N],count=0;
int main(){
	cin>>ptime>>n;
	for(int i=1;i<=n;i++){
		cin>>start[i]>>pend[i];
	}
	for(int i=1;i<=ptime;i++){//ptime=500
		for(int j=1;j<=n;j++){//n=3
			if(start[j]==i){
				//cout<<"find"<<start[j]<<endl;
				for(int m=start[j];m<=pend[j];m++){
					flag[m]++;
				}
			}
		}
	}
	for(int i=1;i<=ptime;i++){
		if(flag[i]>0){
			ptime--;
		}
	}
	cout<<ptime-1;
	return 0;
}