#include <fstream>
#include <algorithm>
using namespace std;
class farmer
{
public:
int price,amount;
};
int cmp(const farmer a,const farmer b)
{
if(a.price<b.price)
return 1;
else
return 0;
}
int main()
{
ifstream fin("milk.in");
ofstream fout("milk.out");
farmer num[5001];
int n,i,sum,money=0;
fin>>n>>sum;
for(i=1;i<=sum;i++)
fin>>num[i].price>>num[i].amount;
sort(num+1,num+sum+1,cmp);	
for(i=1;i<=sum;i++)
{
if(num[i].amount>=n)
{
money+=num[i].price*n;
break;
}
else
{
n=n-num[i].amount;
money+=num[i].price*num[i].amount;
}
}
fout<<money<<endl;
fin.close();
fout.close();
return 0;
}