#include <bits/stdc++.h>
using namespace std;
int main(){
    int l , m , a , b , count = 0;
    
    cin >> l >> m ;
    int arr[l] = {} ;
    for (int i = 0 ; i < l ; i ++) arr[i] = 0 ;
    for (int i = 0 ; i < m ; i ++){
        cin >> a >> b ;
        for (int j = a ; j <= b ; j ++){
            arr[j] = 1 ;
        }
    }
    for (int i = 0 ; i < l ; i ++){
    	if (arr[i] == 0 ){
    		count += 1;
		}
	}
    cout << count << endl; 
    return 0;
}