import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc=new Scanner(System.in);
        int n=sc.nextInt(),m=sc.nextInt();
        List<Map<String ,Integer>> l1=new ArrayList<>();
        for (int i=1;i<=m;i++){
            Map<String ,Integer> milk=new HashMap() ;
            milk.put("单价",sc.nextInt());
            milk.put("数量",sc.nextInt());
            l1.add(milk);
        }
        Collections.sort(l1, new Comparator<Map<String, Integer>>() {
            @Override
            public int compare(Map<String, Integer> o1, Map<String, Integer> o2) {

                    return o1.get("单价")-o2.get("单价");

            }
        });
        int m1=0;
    //    for (int i=0;i<l1.size();i++) {
   //         System.out.println(l1.get(i));
     //   }
        for (int i=0;i<m;i++){
            Map<String, Integer> shop = l1.get(i);
            if(shop.get("数量")>=n){
                m1=m1+n*shop.get("单价");
                n=0;
                break;
            }else {
                m1=m1+shop.get("数量")*shop.get("单价");
                n=n-shop.get("数量");;
            }
        }
        System.out.println(m1);
    }
}
/**************************************************************
	Problem: 1940
	User: admin
	Language: Java
	Result: Accepted
	Time:4429 ms
	Memory:54228 kb
****************************************************************/