FatMouse' Trade

FatMouse prepared M pounds of cat food, ready to trade with the cats guarding the warehouse containing his favorite food, JavaBean.
The warehouse has N rooms. The i-th room contains Jii pounds of JavaBeans and requires Fii pounds of cat food. FatMouse does not have to trade for all the JavaBeans in the room, instead, he may get Jii* a% pounds of JavaBeans if he pays Fii* a% pounds of cat food. Here a is a real number. Now he is assigning this homework to you: tell him the maximum amount of JavaBeans he can obtain.
Input

The input consists of multiple test cases. Each test case begins with a line containing two non-negative integers M and N. Then N lines follow, each contains two non-negative integers Jii and Fii respectively. The last test case is followed by two -1's. All integers are not greater than 1000.

Output

For each test case, print in a single line a real number accurate up to 3 decimal places, which is the maximum amount of JavaBeans that FatMouse can obtain.

Sample Input

5 3
7 2
4 3
5 2
20 3
25 18
24 15
15 10
-1 -1

Sample Output

13.333

31.500

贪心，一直贪性价比最高的，很奇怪我用cin一直是wa，怀疑cin和scanf读入的精度不一样

#include<algorithm>
#include<stdio.h>
#include<iostream>
#include<iomanip>
using namespace std;
struct node
{
    double x,y;
    double v;
};
node a[1005];
bool cmp(node a,node b)
{
    return a.v>b.v;
}
int main()
{
    double sum,ans;
    int n,k,i;
    while(scanf("%lf%d",&sum,&n))
    {
        if (sum==-1&&n==-1) break;
        ans=0;
        k=0;
        for(i=0;i<n;i++)
        {
            cin>>a[i].x>>a[i].y;
            a[i].v=a[i].x/a[i].y;
        }
        sort(a,a+n,cmp);
        while(sum>0&&k<n)
        {
            if (sum>a[k].y)
            {
                sum=sum-a[k].y;
                ans=ans+a[k].x;
                k++;
            }
            else
            {
                ans=ans+sum/a[k].y*a[k].x;
                break;
            }
        }
        printf("%.3lf\n",ans);
    }
    return 0;
}