hdu 1009

FatMouse’ Trade

 #include <cstdio>
#include <iostream>
#include <algorithm>
#include <cstring>
using namespace std;
struct room{
	int beans;
	int foods;
	double ave;
}ro[100000];
bool compare(room a,room b){
	return a.ave>b.ave;
}
int main()
{
	int m,n;
	while(scanf("%d %d",&m,&n)!= EOF){
	if(m==-1 && n==-1) break;
		memset(ro,0,sizeof(ro));
		for(int i=1;i<=n;i++){
			scanf("%d%d",&ro[i].beans ,&ro[i].foods );
			ro[i].ave =ro[i].beans *1.0/ro[i].foods ;
		}
		sort(ro+1,ro+n+1,compare);
		double sum=0.0;
		for(int i=1;i<=n;i++){
			if(m<=0)
				break;
			if(m>=ro[i].foods ){
				sum+=ro[i].beans ;
				m-=ro[i].foods ;
			}
			else{
				sum+=m*1.0/ro[i].foods *ro[i].beans ;
				break;
			}
		}
		printf("%.3lf\n",sum);
	}
	return 0;
 }