#include<bits/stdc++.h>
using namespace std;

const int milk=2000001,fm=5001;
pair<int,int>f[fm];
int cmp(pair<int ,int>a,pair<int ,int >b)
{
    return a.first<b.first;
}
int main()
{int total;
cin>>total;
int shuju;
cin>>shuju;
int danjia,shuliang;
for(int i=1;i<=shuju;i++)
{
    cin>>f[i].first>>f[i].second;
}
    sort(f+1,f+shuju+1,cmp);
int zongjiage=0;
 for(int i=1;i<=shuju;i++)
{
    if(total>f[i].second)
    {zongjiage+=f[i].second*f[i].first;
    total-=f[i].second;

    }
    else if(total<=f[i].second)
    {
       zongjiage+=total*f[i].first;
    break;

    }
}   
   cout<<zongjiage<<endl; 
}