#include <fstream>
#include <algorithm>
using namespace std;
ifstream cin("milk.in");
ofstream cout("milk.out");
class hhh
{
    public:
	int jg,zl;
};
int main()
{
	int x; hhh gb[10001];int i,j,mx=0,r;
	cin>>x>>r;
	for(i=1;i<=r;i++)
		cin>>gb[i].jg>>gb[i].zl;
	for(i=1;i<=r-1;i++)
		for(j=i+1;j<=r;j++)
			if(gb[i].jg<gb[j].zl)
		{
			int t,th;
			t=gb[i].jg;
			th=gb[i].zl;
			gb[i].jg=gb[j].jg;
			gb[i].zl=gb[j].zl;
			gb[j].jg=t;
			gb[j].zl=th;
		}
			for(i=1;i<=r;i++)
			{
		        int nn;
				nn+=gb[i].zl;
				mx+=gb[i].jg*gb[i].zl;
				if(nn==x)
				    break;
				if(nn>x)
				{
					int f;
					f=nn-x;
					mx=mx-gb[i].jg*f;
					break;
				}
				
			}cout<<mx<<endl;				
	cin.close();cout.close();
	return 0;
}