HDU 1009贪心

#include <iostream>
#define MAX 999999999
using namespace std;
double J[1000]={0},F[1000]={0};
double R[1000]={0};
int main()
{
	int M,N;
	double a;
	double JavaBean,CatFood;
	while(cin>>M>>N){
		if(M==-1&&N==-1) break;
		JavaBean=0;
		CatFood=0;
		for(int i=0;i<N;i++){
			cin>>J[i]>>F[i];
			if(F[i]==0) R[i]=MAX;
				else  R[i]=J[i]/F[i];
		}
		
		//sort
		for(int i=0;i<N-1;i++)
			for(int j=i+1;j<N;j++)
				if(R[i]<R[j]) {
					swap(R[i],R[j]);
					swap(F[i],F[j]);
					swap(J[i],J[j]);
				}
		
		for(int i=0;i<N;i++){
			CatFood+=F[i];
			JavaBean+=J[i];
			if(CatFood>M){
				JavaBean-=J[i];
				CatFood-=F[i];
				a=(M-CatFood)/F[i];
				JavaBean+=J[i]*a;
				break;
			}
		}	
		printf("%.3f\n",JavaBean);
	}
	return 0;
}