n,k = map(int,input().split())
*s, = map(int,input())
INF = 1000
nxt = [0]*n
val = [0]*n
for i in range(n):
    v = s[i]
    ii = i+1
    while 0 < v < 200:
        v -= 1
        v += s[ii%n]
        ii += 1
    nxt[i] = ii%n
    val[i] = ii-i if v==0 else k+1

nxt = [nxt]
val = [val]
for _ in range(32):
    nn = [nxt[-1][nxt[-1][i]] for i in range(n)]
    nxt.append(nn)
    nn = [val[-1][i] + val[-1][nxt[-1][i]] for i in range(n)]
    val.append(nn)

def check(x): #x回遷移でval>=k?
    v = idx = 0
    for i in range(31):
        if x>>i&1:
            v += val[i][idx]
            idx = nxt[i][idx]
    return v >= k


ng = 0
ok = k
while ok-ng>1:
    mid = (ok+ng)//2
    if check(mid): ok = mid
    else: ng = mid
print(ok)