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很簡單的貪心，寫一個結構體，按j/f的比例從大到小排，有限選擇比例大的就行，注意要用double，float可能精度不夠第一次WA了。

#include <iostream>
#include <cstdio>
#include <cstring>
#include <string>
#include <map>
#include <algorithm>
#include <cmath>
#include <iomanip>
#include <vector>
#include <stack>
#include <cstdlib>
#include <queue>
#include <set>
#include <cctype>
using namespace std;
#define  loop(i,u,v) for(int i = u;i <= v;i++)
#define rloop(i,u,v) for(int i = u;i >= v;i--)
#define eps  0.00000001
typedef long long ll;

struct stu
{
    int x;
    int y;
} jf[10001];

bool cmp(stu a,stu b)
{
    float rate1 = (float)a.x / a.y;
    float rate2 = (float)b.x / b.y;
    return rate1 > rate2;
}

int main()
{
    int n,m;
    while(cin >> n >> m)
    {
        if(n == -1)
            break;
        loop(i,1,m)
            scanf("%d%d",&jf[i].x,&jf[i].y);

        sort(jf+1,jf+1+m,cmp);
        double sum = 0; 
        loop(i,1,m)
        {
            if(n <= 0)
                break;
            else if((double)n >= jf[i].y)
            {
                sum += (double)jf[i].x;
                n -= jf[i].y;
            }
            else
            {
                sum += (double) jf[i].x / jf[i].y * n;
                break;
            }
        }
        printf("%.3lf\n",sum );
    }
    return 0;
}