while True:
    try:
        x, n = map(int, (input().split()))
        if x == n == -1:
            break
        drink = [0 for i in range(n)]
        for i in range(n):
            drink[i] = list(map(int, input().split()))
        drink.sort(key=lambda x: x[1] / x[0])

        ans = 0
        for i in range(n):
            if x >= drink[i][1]:
                x -= drink[i][1]
                ans += drink[i][0]
                # print(drink[i][0])
            else:
                t = x * drink[i][0] / drink[i][1]
                ans += t
                # print(t)
                break
        print("%.3f" % ans)
    except:
        break