excel problems

How to make the A5 is the most restrictive import value.
Note:A5 = A1 to A4 as the sum of the A5 is the largest calculation of the number 50, when more than 50 are not allowed to enter

cell A5 set of data for effective “small”
data “between”
minimum -99999999999999999
max = sum (a1:a4)

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that you do not know More than 50 are not allowed to enter on the A5 is not empty.= IF (SUM (A1:A4)> 50, “data error”, SUM (A1:A4)) You mean like a mistake, like a warning to prohibit A1 A4 and enter the total number of 50 for?