Find Missing Number

a list numbers which content from 1..xx, but it missing one number,find it

SUM range( 1 - n )

$$\begin{gathered} 1+2+3+4+..n \\ n+(n-1)+(n-2)+(n-3)..1 \\ (n+1)\ast n \\ . \\ . \\ sum(\mathrm{1..}n)=(n+1)\ast n/2 \end{gathered}$$

table