Excel 2016: A nested if statement error is coming up with the correct # of parenthesis

IF(C5="","",IF(D5="",C5,IF(E5="",(SUM(C5:D5/2),IF(F5="",(SUM(C5:F5/3),IF(G5="",(SUM(C5:F5/4),IF(H5="",(SUM(C5:G5/5),IF(I5="",(SUM(C5:H5/6),IF(J5="",(SUM(C5:I5/7),IF(K5="",(SUM(C5:J5/8),IF(L5="",(SUM(C5:K5/9),(SUM(C5:L5/10))))))))))))))))))))

1.Trying to find an average based on number of trials(columns)