import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;

/**
 * @author: CyS2020
 * @date: 2021/4/29
 * 描述：区间覆盖
 */
public class Main {

    public static void main(String[] args) throws IOException {
        BufferedReader input = new BufferedReader(new InputStreamReader(System.in));
        String line = input.readLine();
        int[] arr = Arrays.stream(line.split(" ")).mapToInt(Integer::parseInt).toArray();
        int st = arr[0];
        int ed = arr[1];
        line = input.readLine();
        int N = Integer.parseInt(line);
        List<Pair> intervals = new ArrayList<>();
        while ((line = input.readLine()) != null) {
            arr = Arrays.stream(line.split(" ")).mapToInt(Integer::parseInt).toArray();
            intervals.add(new Pair(arr[0], arr[1]));
        }

        Collections.sort(intervals);

        int cur = Integer.MIN_VALUE;
        int res = 0;
        int i = 0;
        while(cur < ed){
            for(; i < intervals.size(); i++){
                Pair pair = intervals.get(i);
                if(pair.left <= st){
                    cur = Math.max(cur, pair.right);
                }else {
                    break;
                }
            }
            if(cur == st){
                res = -1;
                break;
            }
            st = cur;
            res++;
        }
        System.out.println(res);
    }

    public static class Pair implements Comparable<Pair> {
        int left;
        int right;

        public Pair(int left, int right) {
            this.left = left;
            this.right = right;
        }

        @Override
        public int compareTo(Pair pair) {
            return this.left - pair.left;
        }
    }
}