#include<iostream>
#include<vector>
#include<utility>
#include<fstream>

using namespace std;

class piggy_bank{
	private: int weight;
		 int null_weight;
		 int coins_cout;
		 vector<pair<int,int>> coins;

		 vector<float> coins_wc;
	public:  piggy_bank(int, int, int, vector<pair<int,int>>);
		 void calculate_wc();
		 void out();
		 int result();
};

using namespace std;

int main(){
	ifstream input("input.txt");
	ofstream output("output.txt");

	int weight, null_weight, coins_cout;
	vector<pair<int,int>> coins;
	input >> null_weight >> weight >> coins_cout;
	for ( auto i =0 ; i < coins_cout; i++ ){
		int coin_weight, coin_cost;
		input >> coin_weight >> coin_cost;
		coins.push_back(make_pair(coin_weight,coin_cost));
	}

	piggy_bank bank = piggy_bank(weight, null_weight, coins_cout,coins);
	bank.calculate_wc();
	//bank.out();
	output << bank.result();

	input.close();
	output.close();
	return 0;
}

piggy_bank::piggy_bank(int w, int n, int c, vector<pair<int,int>> co){
	weight = w; null_weight = n; coins_cout = c; coins = co;
}

void piggy_bank::calculate_wc(){
	for ( auto i = 0 ; i < coins_cout; i++ ){
		coins_wc.push_back(float(coins[i].first)/float(coins[i].second));
	}
	coins_wc.push_back(10001);
}

int piggy_bank::result(){
	int result=0;
	int current_weight=0;
	int sim =  weight - null_weight;
	do{
		float min_wc = coins_cout;
		for ( auto i = 0 ; i < coins_cout; i++ ){
			if ( coins_wc[i] < coins_wc[min_wc] ) min_wc = i;
		}
		current_weight += coins[min_wc].second;
		result += coins[min_wc].first;
		//cout << current_weight << endl;
		//cout << result << endl;
		if (coins_wc[min_wc]==10001){
			return -1;
		}
		if (current_weight > sim){
			current_weight -=coins[min_wc].second;
			result -=coins[min_wc].first;
			coins_wc[min_wc] = 10001;
		}
	}while (current_weight!=sim);

	return result;
}

void piggy_bank::out(){
	cout << "Вес пустой копилки:\t" <<null_weight << endl << "Вес копилки:\t" << weight << endl <<"Количество монет\t" << coins_cout << endl;
	for ( auto i = 0 ; i < coins_cout; i++ )
		cout << "Стоимость монеты\t" << coins[i].first << "\t Вес монеты\t" << coins[i].second << "\t Стоимость на вес:\t" <<coins_wc[i]<< endl;
}