while True:
    try:
        x, n = map(int, input().split())
        dict = {}
        dict2 = {}
        if x == -1 and n == -1:
            pass
        else:
            for j in range(n):
                mi, wi = map(int, input().split())
                danjia = wi / mi
                dict.setdefault(mi, danjia)
                dict2.setdefault(mi, wi)
            list1 = sorted(dict.items(), key=lambda x: (x[1], x[0]))
            total_mi = 0  # ml
            total_wi = 0  # money
            for i in range(len(list1)):
                if dict2[list1[i][0]] <= x:
                    total_mi += list1[i][0]
                    x -= dict2[list1[i][0]]
                else:
                    total_mi += (list1[i][0]/dict2[list1[i][0]])*x
                    break
            print('%.3f' % total_mi)
    except:
        break