I always get missing $ when using \begin{equation}

Question

I am currently writing my master's thesis on LaTeX, and I keep getting problems each time I use the \begin{equation} syntax, this is what I'm using

 \begin{equation}

 DTW(X,Y)= \underset{w_x,w_y}{\text{min}}\sum_{k=1}^{K}d(x_{w_x}(k),y_{w_y}(k))

  \end{equation}

and this is what I get

! Missing $ inserted

so far I have read that \begin{equation} already opens a math environment Any help will be appreciated.