#include <stdio.h>
#include <string.h>
#include <math.h>
#include <stdlib.h>
int main()
{
    int l, m;
    scanf("%d %d", &l, &m);
    int a[l];
    for (int i = 0; i < l + 1; i++)
    {
        a[i] = 1;
    } // 初始化坐标的树，1为有，0为无

    for (int i = 0; i < m; i++)
    {
        int begin, end;
        scanf("%d %d", &begin, &end);
        for (int i = begin; i < end + 1; i++)
        {
            a[i] = 0;
        }
    }

    int n = 0;
    for (int i = 0; i < l + 1; i++)
    {
        if (a[i] == 1)
        {
            n++;
        }
    }

    printf("%d", n);
    return 0;
}