C :

#include<stdio.h>
#include<string.h>
typedef struct{
    double W;
    double F;
    double Val;
}Food;
Food food[1000];
int main()
{
    int M, N;
    int i,j;
    while(scanf("%d %d",&M,&N))
{
        if(M == -1 && N == -1)
        {
            break;
        }
        memset(food,0,sizeof(food));
        for(i=0;i<N;i++)
        {
            scanf("%lf %lf",&food[i].W,&food[i].F);
            food[i].Val = food[i].W/food[i].F;
        }
        //sort(food,food+N,Cmp);
        for(i=0;i<N;i++)
        {
            for(j=i+1;j<N;j++)
            {
               if(food[i].Val<food[j].Val)
               {
                   Food temp;
                    temp.W=food[i].W;
                    temp.F=food[i].F;
                    temp.Val=food[i].Val;
                    food[i].W=food[j].W;
                    food[i].F=food[j].F;
                    food[i].Val=food[j].Val;
                    food[j].W=temp.W;
                    food[j].F=temp.F;
                    food[j].Val=temp.Val;

               }

            }

        }
        double res = 0;
        for(i=0;i<N;i++)
        {
            if(M >= food[i].F)
            {
                M -= food[i].F;
                res += food[i].W;
            }
            else if(M == 0)
            {
                break;
            }
            else
            {
                double temp = M/food[i].F;
                res += food[i].W*temp;
                M = 0;
            }
        }
        printf("%.3lf\n",res);
    }
    return 0;
}

C++ :

#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
using namespace std;
typedef struct{
    double W;
    double F;
    double Val;
}Food;

Food food[1000];
int Cmp(Food X, Food Y)
{
    return X.Val > Y.Val;
}

int main()
{
    int M, N;
    int i;
    while(scanf("%d %d",&M,&N))
    {
        if(M == -1 && N == -1)
        {
            break;
        }
        memset(food,0,sizeof(food));
        for(i=0;i<N;i++)
        {
            scanf("%lf %lf",&food[i].W,&food[i].F);
            food[i].Val = food[i].W/food[i].F;
        }
        sort(food,food+N,Cmp);
//        for(i=0;i<N;i++)
//        {
//            printf("%lf %lf %lf\n",food[i].W,food[i].F,food[i].Val);
//        }
        double res = 0;
        for(i=0;i<N;i++)
        {
            if(M >= food[i].F)
            {
                M -= food[i].F;
                res += food[i].W;
            }
            else if(M == 0)
            {
                break;
            }
            else
            {
                double temp = M/food[i].F;
                res += food[i].W*temp;
                M = 0;
            }
        }
        printf("%.3lf\n",res);
    }
    return 0;
}