/*

ID:kevin_s1

PROG:milk

LANG:C++

*/

#include <iostream>

#include <string>

#include <cstring>

#include <cstdio>

#include <algorithm>

#define MAXN 5001

using namespace std;

int N,M;

struct farmer{

	int Pi;

	int Ai;

}farmers[MAXN];

int cmp(farmer a,farmer b){

	return a.Pi < b.Pi;

}

int main(){

	freopen("milk.in","r",stdin);

	freopen("milk.out","w",stdout);

	cin>>N>>M;

	for(int i = 1; i <= M; i++){

		cin>>farmers[i].Pi>>farmers[i].Ai;

	}

	sort(farmers + 1, farmers + M + 1, cmp);

	int count = 0;

	int index = 1;

	int money = 0;

	while(count < N){

		if(count + farmers[index].Ai <= N){

			count = count + farmers[index].Ai;

			money += farmers[index].Pi * farmers[index].Ai;

			index++;

		}

		if(count + farmers[index].Ai > N){

			money += farmers[index].Pi * (N - count);

			count = N;

			index ++;

		}

	}

	cout<<money<<endl;

	return 0;

}

//greedy algorithm