N, A = list(map(int,input().split()))

S = input()

ok = 0
for i in range(N):
	if S[i] == "0":
		ok = i
		break
ans = i
T = []
for i in range(2 * N):
	T.append(S[(i + ok) % N])

L = []
cnt = 0

for i in range(2 * N):
	if T[i] == "0":
		L.append(0)
		cnt += 1
	else:
		if cnt > 0:
			L.append(2)
			cnt -= 1
		else:
			L.append(1)

check = []
for i in range(N):
	if L[i] == 2:
		check.append(i + 1)
	elif L[i + N] == 2:
		check.append(i + 1)
		A += 1
	else:
		continue

A -= 1

ans += N * (A // len(check))

A = A % len(check)

ans += check[A]

print(ans)