#include "stdio.h"
int main()
{
    int lenght=0,member=0;
    int a[100][2]={0};
    int i;int j;

    while(!lenght)
    {
        scanf("%d",&lenght);
        if(lenght>=1&&lenght<=10000)
            break;
        else
            lenght =0;
    }
    while(!member)
    {
        scanf("%d",&member);
        if(member>=1&&member<=10000)
            break;
        else
            member =0;
    }

    for(i=0;i<member;i++)
    {
        while(!a[i][0])
        {
            scanf("%d",&a[i][0]);
            if(a[i][0]>=1&&a[i][0]<=lenght)
                break;
            else
                a[i][0]=0;
        }

        while(!a[i][1])
        {
            scanf("%d",&a[i][1]);
            if(a[i][1]>=a[i][0]&&a[i][1]<=lenght)
                break;
            else
                a[i][1]=0;
        }
    }

    //main code
    int b[10001]={0};
    for(i=0;i<=lenght;i++)
    {
        b[i]=1;

        for(j=0;j<member;j++)
        {

            if(i>=a[j][0]&&i<=a[j][1])
            {
                b[i]=0;
            }
        }
    }

    int n=0;
    for(i=0;i<=lenght;i++)
        if(b[i]==1)
            n++;

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