#include <bits/stdc++.h>

#define        READ         freopen("in.txt", "r", stdin);

#define     mx           100005

using namespace std;

double value[mx], weight[mx];

struct val
{
	double per, kg;
}item[mx];

bool compare(val a, val b)
{
	return a.per>b.per;
}

int main()
{
  	//READ
	double need, ans=0;
	int n;

	cin >> need >> n;

	need = need*need*need;

	for(int i = 0; i < n; i++)
	{
		cin >> weight[i];
		cin >> value[i];
	}

	//for(int i = 0; i < n; i++)

	for(int i = 0; i < n; i++)
	{
		item[i].per = value[i]/weight[i];
		item[i].kg = weight[i];
	}

	sort(item, item+n, compare);

	for(int i = 0; i < n; i++)
	{
		if(need==0) break;
		else if(item[i].kg<=need)
		{
			need -= item[i].kg;
			ans += (item[i].kg*item[i].per);
		}
		else
		{
			ans += (item[i].per*need);
			need = 0;
		}
	}

	cout << ans << endl;


	return 0;
}