HDOJ 1009

import java.util.Arrays;
import java.util.Scanner;
public class Main {
    public static void main(String[] args)
    {
        Scanner scanner=new Scanner(System.in);
        short M=0,N=0,j,f;
        Food[] fs;
        while(true)
        {
            M=scanner.nextShort();
            N=scanner.nextShort();
            if(N==-1 && M==-1)
                break;
            fs=new Food[N];
            for(short i=0;i<N;i++)
            {
                fs[i]=new Food(scanner.nextShort(),scanner.nextShort());
            }
            Arrays.sort(fs);//注意这里的N和String的subString的endIndex一样都不包含在内
            float sum=0;
            short i=0;
            while(M>0 && i<N)
            {
                if(fs[i].f>=M)
                {
                    sum+=M*fs[i].j*1.0f/fs[i].f;
                    M=(short)(M-M);
                }
                else {
                    sum+=fs[i].j;
                    M=(short)(M-fs[i].f);
                }
                i++;
            }
            System.out.println(String.format("%.3f", sum));
        }
    }
    static class Food implements Comparable<Food>
    {
        public short j;
        public short f;
        
        public Food(short j,short f)
        {
            this.j=j;
            this.f=f;
        }
        @Override
        public int compareTo(Food o) {
            // TODO Auto-generated method stub
            if(this.j*1.0f/this.f>=o.j*1.0f/o.f)
                return -1;
            else
                return 1;
        }
    }
}