The function ISNUMBER outputs TRUE if the input is a Number.
ISNUMBER(value) -> result
value (required, type:
The value that may or may not be a Number.
The result of checking if the given input is a Number; TRUE means that it is and FALSE means that it isn't.
The following example checks to see if the number 1.5 is a number. It is:
ISNUMBER(1.5) -> TRUE
The following example checks to see if the string "1.5" is a number. It isn't:
ISNUMBER("1.5") -> FALSE