#include<bits/stdc++.h>

using namespace std;

typedef pair<int, int> PII;

const int N = 100005;

PII p[N];

int main(){
    int s, t, n, cnt = 0;
    cin>>s>>t;
    cin>>n;
    for(int i = 0; i < n; i++){
        cin>>p[i].first>>p[i].second;
    }
    sort(p, p + n);
    for(int i = 0; i < n && s < t; i++){
        int st = s;
        while(i < n && p[i].first <= s){
            st = max(st, p[i].second);
            i++;
        }
        if(s == st){
            cnt = -1;
            break;
        }
        s = st;
        cnt++;
        i--;
    }
    if(s < t)   cnt = -1;
    cout<<cnt<<endl;
    return 0;
}