def main():
    n, a = map(int, input().split())
    S = input()
    t0 = S.count("1")
    t1 = n - t0
    #if t0 >= t1:
    cnt = 0
    now = 0
    for i in range(n):
        if S[i] == "0":
            now -= 1
        elif now < 0:
            cnt += 1
            now += 1
            if cnt == a:
                return i+1
    a -= cnt
    lst = []
    cnt = 0
    for i in range(n):
        if S[i] == "0":
            now -= 1
        elif now < 0:
            cnt += 1
            now += 1
            lst.append(i+1)
    ans = ((a-1) // cnt + 1) * n
    return ans + lst[(a-1) % cnt]
        

print(main())