HDOJ-1009 FatMouse' Trade

# include <stdio.h>
# include <algorithm>
# define MAX 1010
using namespace std;

struct NODE
{
	int j, f;
	double avg;
}food[MAX];

int n, m;

int cmp(const NODE& a, const NODE& b)
{
	return a.avg > b.avg;
}

int main()
{
	while(scanf("%d %d", &m, &n) && m != -1 && n != -1)
	{
		for(int i = 0; i < n; i++)
		{
			scanf("%d %d",&food[i].f, &food[i].j);
			food[i].avg = food[i].f * 1.0 / food[i].j;
		}sort(food, food+n, cmp);

		double ans = 0;
		for(int i = 0; i < n; i++)
		{
			if(m > food[i].j)
			{
				ans += food[i].f;
				m -= food[i].j;
			}
			else
			{
				ans += (m * 1.0 / food[i].j * food[i].f);
				m = 0;
				break;
			}
		}
		printf("%.3lf\n", ans);
	}
	
	return 0;
}