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

hit = 0
for i in range(N):
    hit += int(S[i])
    
if hit >= N:
    rest = 0
    ans = 0
    for i in range(len(S)):
        if i >= K:
            break
        rest -= 1
        while rest < 0:
            rest += 1
            ans += 1
        rest += int(S[i])
    print(ans)
else:
    ans = K
    ans -= K // len(S) * hit
    for i in range(K % len(S)):
        ans -= int(S[i])
    waste = 0
    tmp = 0
    chg = 0
    for i in range(K - 1, -1, -1):
        if chg >= 100:
            break
        tmp += int(S[i % len(S)])
        if tmp > waste:
            waste = tmp
            chg = 0
        tmp -= 1
        chg += 1
    ans += waste
    print(ans)