#include <iostream>
#include <fstream>
#include <algorithm>
using namespace std;
ifstream f("acoperire.in");
ofstream g("acoperire.out");
pair<int,int> a[1005];
int x,y,n,i,sol,start,finish;
int main()
{
    f>>x>>y;
    f>>n;
    for(i=1;i<=n;i++)
    {
        f>>a[i].first>>a[i].second;
    }
    a[n+1].first=10005;
    a[n+1].second=10005;
    n=n+1;
    sort(a+1,a+n+1);
    start=x;
    finish=x-1;
    i=1;
    while(i<=n)
    {
        //cout<<a[i].first<<" ";
        if(a[i].first<=start)
        {
            finish=max(finish,a[i].second);
            i++;
        }
        else
        {
            start=finish;
            //cout<<i-1<<" ";
            sol++;
            if(a[i].first>finish || finish>=y)
            {
                break;
            }
        }
    }
    //cout<<finish<<" ";
    if(finish<y)
    {
        g<<-1;
    }
    else
    {
        g<<sol;
    }
    return 0;
}