Painting Fence Sections

You are tasked with painting a fence with alternating colors of white and black. The fence is divided into sections, each represented by a block. Each block can either be white ('W') or black ('B').

Your goal is to ensure that there is at least one occurrence of a desired number of consecutive black blocks, denoted by k(a number on the first index of string). However, initially, some blocks may be white.

In one operation, you can repaint a white block to make it black.

Your task is to determine and print the minimum number of painting operations needed to achieve the goal of having at least one occurrence of k consecutive black blocks.