import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int m, n;
        m = sc.nextInt();
        n = sc.nextInt();
        List<Map<String, Integer>> shopList = new ArrayList<>();
        for (int i = 1; i <= n; i++) {
            Map<String, Integer> shop = new HashMap<>();
            shop.put("price", sc.nextInt());
            shop.put("count", sc.nextInt());
            shopList.add(shop);
        }
        //容器排序
        Collections.sort(shopList, new Comparator<Map<String, Integer>>() {
            @Override
            public int compare(Map<String, Integer> o1, Map<String, Integer> o2) {
                //如果前一个 price > 后一个 price，则交换这两个map
                return o1.get("price") - o2.get("price");
            }
        });
        //总共需要购买 m 个贺卡，m 还需要购买的贺卡个数
        int money = 0;
        for (int i = 0; i < shopList.size(); i++) {
            //第 i 间商铺
            Map<String, Integer> shop = shopList.get(i);
            if( shop.get("count") >= m){
                //不需要这么多了
                money = money + m * shop.get("price");
                m = 0;
                break;
            }else{
                money = money + shop.get("count") * shop.get("price");
                m = m - shop.get("count");
            }
        }
        System.out.print(money);
        sc.close();
    }
}

/**************************************************************
	Problem: 1730
	User: admin
	Language: Java
	Result: Accepted
	Time:2253 ms
	Memory:40664 kb
****************************************************************/