/*
ID: mycodeb1
LANG: C++
TASK: milk
*/
 
#include <bits/stdc++.h>
using namespace std;
const int MAXN = 5000 + 100;
 
struct PRICE
{
    int per, amount;
    bool operator < (const PRICE &a) const
    {
        return per < a.per;
    }
}far[MAXN];
 
int main()
{
    //freopen("input.txt", "r", stdin);
    freopen("milk.in", "r", stdin);
    freopen("milk.out", "w", stdout);
 
    int need, i, j, n, ans;
    scanf("%d%d", &need, &n);
    for (i = 0; i < n; i++)
        scanf("%d%d", &far[i].per, &far[i].amount);
    sort(far, far + n);
    ans = 0;
    for (i = 0; i < n; i++)
    {
        if (need >= far[i].amount)
        {
            need -= far[i].amount;
            ans += far[i].per * far[i].amount;
        }
        else
        {
            ans += far[i].per * need;
            break;
        }
    }
    printf("%d\n", ans);
    return 0;
}