C :

#include <stdio.h>
#include <stdlib.h>
//#include <string.h>
int main(void)
{
	int L,M;
	int a,b;
	int *p;
	int i;
	int count=0;
	while((scanf("%d%d",&L,&M),L))
	{
		p=(int *)malloc(sizeof(int)*(L+1));
		//memset(p,0,L+1);
		for(i=0;i<L+1;i++)
		{
			p[i] = 0;
		}
		while(M)
		{
			scanf("%d%d",&a,&b);
			for(i=a;i<=b;i++)
			{
				p[i] = 1;
			}
			M--;
		}
		count = 0;
		for(i=0;i<L+1;i++)
		{
			if(p[i] == 0)
			{
				count++;
			}
		}
		free(p);
		printf("%d\n",count);
	}
	return 0;
}

C++ :

#include <stdio.h>
int l,m;
int run()
{
	int a[11111],i,j,k,h,t=0;
	for(i=0;i<=l;i++)
		a[i]=1;
	for(i=1;i<=m;i++)
	{
		scanf("%d%d",&j,&k);
		for(h=j;h<=k;h++)
			a[h]=0;
	}
	for(i=0;i<=l;i++)
		t+=a[i];
	return t;
}
int main()
{
	scanf("%d%d",&l,&m);
	while(l!=0)
	{
		printf("%d\n",run());
		l=0;
		scanf("%d%d",&l,&m);
	}
	return 0;
}

Java :

import java.util.Scanner;

public class Main {
	
	public static void main(String[] args) {
		Scanner scanner = new Scanner(System.in);
		while(scanner.hasNext()){
			int l=scanner.nextInt();
			if(l==0){
				continue;
			}
			int m=scanner.nextInt();
			boolean[] tree=new boolean[10010];
			while(m--!=0){
				int begin=scanner.nextInt();
				int end=scanner.nextInt();
				for (int i = begin; i <= end; i++) {
					tree[i]=true;
				}
			}
			int ans =0;
			for (int i = 0; i <= l; i++) {
				if(tree[i]==false){
					++ans;
				}
			}
			System.out.println(ans);
		}
		
	}

}

C# :

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace 剩下的树
{
    class Program
    {
        static void Main(string[] args)
        {
            string s = Console.ReadLine();
            string[] words = new string[100];
            words=s.Split(' ');
            int l = int.Parse(words[0]), m = int.Parse(words[1]);
            while (l != 0 || m != 0)
            {
                bool[] exist = new bool[10001];
                for (int i = 0; i <= l; i++)
                {
                    exist[i] = true;
                }
                while (m-- != 0)
                {
                    s = Console.ReadLine();
                    words = s.Split(' ');
                    int a = int.Parse(words[0]), b = int.Parse(words[1]);
                    for (int i = a; i <= b; i++)
                    {
                        exist[i] = false;
                    }
                }
                int cnt=0;
                for (int i = 0; i <= l; i++) 
                {
                    if (exist[i])
                        cnt++;
                }
                Console.WriteLine("{0}", cnt);
                s = Console.ReadLine();
                words = new string[100];
                words = s.Split(' ');
                l = int.Parse(words[0]);m = int.Parse(words[1]);
            }
        }
    }
}