法一：暴力法，bool标记1和0之后进行统计
此处代码略······
法二：区间合并，引用PIIs直接区间首尾相减得到每个区间拔掉的树

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.Scanner;

public class 部落外的树 {
    
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        //合并区间问题
        List<PIIs> list = new ArrayList<PIIs>();
        Scanner scan = new Scanner(System.in);
        int T = scan.nextInt();
        int n = scan.nextInt();
        int m = scan.nextInt();
        for(int i=0;i<m;i++) {
            int l = scan.nextInt();
            int r = scan.nextInt();
            list.add(new PIIs(l,r));
        }
        Collections.sort(list);
        //初始化边界为无穷
        int start = Integer.MIN_VALUE;
        int end = Integer.MIN_VALUE;
        int []left = new int[m];
        int []right= new int[m];
        for(int i=0;i<list.size();i++) {
            left[i]=((PIIs) list).getFirst();
            right[i]=((PIIs) list).getSecond();
        }
        for(int i=1;i<m;i++) {
            if(left[i]<right[i-1]) {
                left[i]=left[i-1];
            }
            else {
                int x=right[i-1]-left[i-1]+1;
                n-=x;
            }
        }
        int x=right[m-1]-left[m-1]+1;
        n-=x;  //减去最后一个区间
        System.out.println(n);
    }

}
class PIIs implements Comparable<PIIs>{
    private int first, second;

    public PIIs (int first, int second) {
        this.first = first;
        this.second = second;
    }

    public int getFirst() {
        return first;
    }

    public int getSecond() {
        return second;
    }

    @Override
    public int compareTo(PIIs object) {
        return Integer.compare(first, object.first);
    }

}