#coding:utf-8

N,K=map(int,raw_input().split())
S=raw_input()

cnt_l,ans_l=0,0
cnt_r,ans_r=0,0

if K <= len(S)/2:	
	for i in S[K-1:]:
		cnt_l+=1
		if i == S[K-1]:
			ans_l+=1
		else:
			ans_l-=1
			if ans_l == 0:
				print cnt_l+(K-1)
				break
else:
	for i in S[:K][::-1]:
		cnt_r+=1
		if i == S[K-1]:
			ans_r+=1
		else:
			ans_r-=1
			if ans_r == 0:
				print cnt_r-(K-1)
				break