/*

ID:wang9621

PROG:milk

LANG:C++

*/

#include <iostream>

#include <cstdio>

#include <algorithm>

using namespace std;

struct edge

{

    int cent;

    int unit;

};

edge per[];

bool cmp(edge a,edge b)

{

    return a.cent<b.cent;

}

int main()

{

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

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

    int n,m;

    scanf("%d %d",&n,&m);

    for(int i = ; i<=m; i++)

    {

        scanf("%d %d",&per[i].cent,&per[i].unit);

    }

    sort(per+,per+m+,cmp);

    int sum = ;

    for(int i = ; i<=m; i++)

    {

        if(n>=per[i].unit)

        {

            sum += per[i].cent*per[i].unit;

            n -= per[i].unit;

        }

        else if(n>)

        {

            sum += n*per[i].cent;

            n = ;

        }

    }

    printf("%d\n",sum);

    return ;

}