import java.util.*;

public class Main{
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        int l = sc.nextInt();
        int r = sc.nextInt();
        int n = sc.nextInt();
        List<int[]> list = new ArrayList();
        while(n-->0){
            int[] arr = new int[2];
            arr[0] = sc.nextInt();
            arr[1] = sc.nextInt();
            list.add(arr);
        }
        Collections.sort(list, new Comparator<int[]>(){

            public int compare(int[] o1,int[] o2){
                return o1[0]-o2[0];
            }
        });
        int idx = 0;
        int ans = 0;
        while(l<r){
            int t = -1;
            int max = l;
            while(idx<list.size()){
                int[] arr = list.get(idx);
                if(arr[0]<=l && arr[1]>=l){
                    if(arr[1]>max){
                        t = idx;
                        max = arr[1];
                    }
                }else if(arr[0]>l){
                    break;
                }
                idx++;
            }
            if(t == -1){
                System.out.println(-1);
                return;
            }
            ans++;
            l = list.get(t)[1];
        }
        System.out.println(ans);
    }
}