validinputone = False
while not validinputone:
    inputone = input("What do you want to convert? [x] [km/h|m/s|mph|knots]")

    if " " not in inputone:
        print("Please separate the number and unit with a space.")
        continue

    splits = inputone.split(" ")

    try:
        originalinput = float(splits[0])
        number = float(splits[0])
    except ValueError:
        print("Please enter a number, then a unit.")
        continue

    unit = splits[1]

    if unit == "km/h":
        validinputone = True
        number = number / 3.6
    elif unit == "m/s":
        validinputone = True
        number = number
    elif unit == "knots":
        validinputone = True
        number = number / 1.944
    elif unit == "mph":
        validinputone = True
        number = number / 2.237
    else:
        print("Please provide a valid unit.")

validinputtwo = False
while not validinputtwo:
    inputtwo = input("What unit do you want this converted to? [km/h|m/s|mph|knots]")

    if inputtwo == "km/h":
        validinputtwo = True
        number = number * 3.6
    elif inputtwo == "m/s":
        validinputtwo = True
        number = number
    elif inputtwo == "knots":
        validinputtwo = True
        number = number * 1.944
    elif inputtwo == "mph":
        validinputtwo = True
        number = number * 2.237
    else:
        print("Please provide a valid unit.")

print("{}{} is {}{}.".format(originalinput, unit, number, inputtwo))