#include <stdio.h>
#include <iostream>
#include<cstdio>
using namespace std;

int main()
{
    int a, b;
    int count = 0;
    int s[1000];
    cin >> a >> b;
   
    
    for (int i = 0; i <= a; i++)
    {
        s[i] = 0;
    }
    for (int k = 1; k <= b; k++)
    {
        int head, tail;
        cin >> head >> tail;
        for (int j = head; j <= tail; j++)
        {
            if (s[j] == 0)
            {
                s[j] = 1;
            }
        }
    }
    for (int i = 0; i <= a; i++)
    {
       
        if (s[i] == 0)
        {
            count++;
        }
    }
    cout << count;
    return 0;
}