#include <bits/stdc++.h>
using namespace std;
int n,m;//n--需奶量
int p,a;
long long sum;
int m1;
map <int,int>mp;
int main() {
cin>>n>>m;
for(int i = 1;i <= m;i++)
{
cin>>p>>m1;
mp[p] += m1;
}
for(auto x:mp)
{
if(n >= x.second)
{
sum += x.first * x.second;
n -= x.second;
}
else
{
sum += x.first * n;
break;
}
}
cout<<sum<<endl;
return 0;
}