/*
ID: vvatash1
PROG: numtri
LANG: C++
*/
using namespace std;
#include <algorithm>
#include <numeric>
#include <iostream>
#include <sstream>
#include <string>
#include <vector>
#include <queue>
#include <set>
#include <map>
#include <cstdio>
#include <cstdlib>
#include <cctype>
#include <cassert>
#include <cmath>
#include <complex>
#include <stack>
#include <iostream>
#include <fstream>
#include <vector>
#include <fstream>

int main()
{
	ifstream fin ("board.in");
	ofstream fout ("board.out");
	int numberLength = 0, pieceLenth = 0;
	int brokenNumber =0, brokenStart = 0, brokenLength =0;
	int board[16000]={0};

	fin >> numberLength >> pieceLenth >> brokenNumber;

	for (int i = 0; i < brokenNumber; i++)		
	{
		fin >> brokenStart >> brokenLength;
		for (int j = brokenStart; j < brokenStart+brokenLength; j++)
		{
			board[j] = 1;
		}
	}
	int countPieces = 0;
	int counter = 0;
	for (int i = 0; i < numberLength; i++)
	{
		if(counter == pieceLenth)
		{
			countPieces++;
			counter = 0;
		}
		if(board[i] == 1)
			counter = 0;
		else 
			counter++;
	}

	fout << countPieces << "\n";
	fout.close();
}