C++ :

#include<iostream>
#include <algorithm>
using namespace std;

const int NN = 10001;




void Process(bool trees[], int start, int end)
{
    for (int i=start; i<=end; i++)
        trees[i] = false;
}




int main()
{
    int L, M, i;
    bool trees[NN];

    int start, end;
    while(cin>>L >>M)
    {
        for (i=0; i <= L; i++)
        {
            if (i%2 == 0)
                trees[i] = true;
            else
                trees[i] = false;
        }
        
        for (i=0; i<M; i++)
        {
            cin>>start >>end;

            Process(trees, start, end);
           
        }
        cout<<count(trees, trees+L+1, true)<<endl;
    }
    return 0;
}

Java :

import java.util.Scanner;

public class Main {
	
	public static void main(String[] args) {
		
		Scanner input = new Scanner(System.in);
		while(input.hasNextInt()){
			
			int L = input.nextInt();
			int M = input.nextInt();
			int i,j;
			int[][] area = new int[M][2];
			int[] tree = new int[L+1];
			int treeNum = 0;
			for(i = 0; i < M; i++){
				
				for(j = 0; j < 2; j++){
					
					area[i][j] = input.nextInt();
				}
				for(j = area[i][0]; j <= area[i][1]; j++){
					
					if(j % 2 == 0 && tree[j] != 1){
						
						tree[j] = 1;
						treeNum++;
					}
				}
			}
			
			int treeSum = L / 2 + 1;
			int leftTree = treeSum - treeNum;
			
			System.out.println(leftTree);
			
			
			
		}
	}

}