Sum of consecutive numbers

Let $n$ be a positive integer, such that there exist an other positive integer $k$ ( $k>n$ ), where ${\color{#3D99F6}{1+2+3+\dots+n}}={\color{#D61F06}{(n+1)+(n+2)+(n+3)+\dots+k}}$ If the third smallest possibe value of $n$ is $N$ , and the thrird smallest possible value of $k$ is $K$ , then find the value of $(K-N)^2$