#include <bits/stdc++.h>
using namespace std;
const int N=5010;
typedef  struct node{
    int x,y;
} PII;
PII s[N];

bool cmp(PII a,PII b)
{
    return a.x < b.x;
}

int main()
{
    int n,m;
    cin>>n>>m;
    for(int i=0;i<m;i++){
        int x,y;
        cin>>x>>y;
        s[i]={x,y};   //单价和数目
    }
    sort(s,s+m,cmp);
    int res=0;
    int cnt=0;
    for(int i=0;i<m;i++){
        cnt+=s[i].y;  res+=s[i].x * s[i].y;
        if(cnt>n){
            cnt-=s[i].y;  res-=s[i].x * s[i].y;
            res+=s[i].x * (n-cnt);
            break;
        }
    }
    cout<<res<<endl;
    return 0;
}