HDOJ1009

#include"iostream"
#include"algorithm"
using namespace std;
struct job
{
	double j;
	double c;
	friend bool operator<(job&a,job&b)
	{
		return a.j/a.c>b.j/b.c;
	}
};
int main()
{
	//freopen("a.txt","r",stdin);
	int n,m;
	while(cin>>n>>m&&n!=-1&&m!=-1)
	{
		job a[1000];
		int i;
		for(i=0;i<m;i++)
			scanf("%lf%lf",&a[i].j,&a[i].c);
		sort(a,a+m);
		
		double sum=0;
		double space=n;
		for(i=0;i<m;i++)
		{
			if(space>0)
			{
				if(space-a[i].c>=0)
				{
					sum+=a[i].j;
					space=space-a[i].c;
					continue;
				}
				if(space-a[i].c<0)
				{
					double st=space/a[i].c*a[i].j;
					//cout<<st<<endl;
					sum+=st;
					space=0;
				}
			}
			if(space==0) break;
		}
		printf("%.3f\n",sum);
	}
	return 0;
}