#include <stdio.h>
#include <stdlib.h>
int cmp(const void* a, const void* b);

typedef struct room
{
    int bean;
    int all_cost;
    double average_cost;
};

int main()
{
    int catfood = 0 ,room_number = 0;
    struct room rooms[1000];
    double sum = 0.0;

    while (1)
    {
        sum = 0.0;
        scanf("%d%d", &catfood, &room_number);
        if (catfood == -1 && room_number == -1)
            break;

        for (int i = 0; i < room_number; i++)
        {
            scanf("%d%d", &rooms[i].bean, &rooms[i].all_cost);
            rooms[i].average_cost = 1.0 * rooms[i].all_cost / rooms[i].bean;
        }

        qsort(rooms, room_number, sizeof(struct room), cmp);

        for (int i = 0; i < room_number; i++)
        {

            if (catfood >= rooms[i].all_cost)
            {
                catfood -= rooms[i].all_cost;
                sum += rooms[i].bean;
            }
               
            else
            {
                sum += (1.0 * catfood / rooms[i].all_cost) * rooms[i].bean;
                break;
            }
        }
        printf("%.3f\n", sum);
    }
    return 0;
}

int cmp(const void* a, const void* b)
{
    struct room A = *(struct room*)a;
    struct room B = *(struct room*)b;

    return A.average_cost - B.average_cost > 0.001;

}