N, K = [int(i) for i in input().strip().split(" ")]
S = [int(i) for i in input().strip()]

def buy(S,N,K,A):
    bought = -1
    count = 0
    atari = A
    ice = 0
    if K == 0:
        return 0
    for i in S:
        if i == 0:
            ice += 1
            if atari == 0:
                count += 1
            else:
                atari -= 1
        elif i == 1:
            ice += 1
            if atari == 0:
                count += 1
            else:
                atari -= 1
            atari += 1
        else:
            ice += 1
            if atari == 0:
                count += 1
            else:
                atari -= 1
            atari += 2
        if ice == K:
            bought = count
    if bought == -1:
        bought = count
    return bought, atari

if N >= K:
    print(buy(S,N,K,0))
else:
    bought, atari = buy(S,N,K,0)
    K -= N
    while K >= 0:
        b, atari = buy(S,N,K,atari)
        bought += b
        K -= N
    print(bought)