#include <bits/stdc++.h>
using namespace std;
int main(){
	int len;
	cin>>len;
	int times;
	cin>>times;
	int ha[10005];
	for(int i = 1;i<=len;i++){
		ha[i] = 1;
	}
	int sum = len;
	for(int i = 0;i<times;i++){
		int l,r;
		cin>>l>>r;
		for(int j = l;j<=r;j++){
			if(ha[j] == 1){
				sum--;
				ha[j] = 0;
			}
		}
	}
	cout<<sum;
	return 0;
}