SUM(...)/A1

Category: Math and trigonometry

Adds its arguments

SUM(number1,[number2],...)

Category: Logical

Specifies a logical test to perform

IF(Something is True, then do something, otherwise do something else)

Category: Information

Returns TRUE if the value is a number

ISNUMBER(value)

0

The formula checks if either of the cell has a numeric value, if it has, the value is returned, otherwise "...". The SUM() formula sums the value that is divided by value in cell A1.