#include<algorithm>
#include<iomanip>
#include <ios>
#include<iostream>
#include<vector>
using namespace std;

struct drink
{
	float m;
	float w;
	float good;
};

bool compare(drink a, drink b)
{
	return a.good > b.good;
}

int main()
{
	int x, n;
	while (cin >> x >> n)
	{
		if (x == -1 && n == -1)
			return 0;
		float sum = 0;
		vector<drink> d(n + 1);
		{
			for (int i = 0; i < n; i++)
			{
				cin >> d[i].m >> d[i].w;
				d[i].good = d[i].m / d[i].w;
			}
		}
		sort(d.begin(), d.end(), compare);
		int i = 0;
		for (; i < n; i++)
		{
			if (x > d[i].w)
			{
				x -= d[i].w;
				sum += d[i].m;
			}
			else
			{
				break;
			}
		}
		if (i < n)
			sum += d[i].m * x / d[i].w;
		cout << fixed << setprecision(3) << sum << endl;
	}
	return 0;
}