def speed_to_pace(x):
    """ convert speed in m/s to pace in min/km """
    if x == 0:
        return 0
    else:
        p = 16.6666666 / x  # 1 m/s --> min/km
        return ':'.join([str(int(p)),
                         str(int((p * 60) % 60)).zfill(2)])