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

SUM=[0]
for s in S:
    SUM.append(SUM[-1]+s)

if N<=SUM[-1]:
    OK=N
else:
    OK=K

def calc(x):
    ANS=(x//N)*SUM[-1]
    if x%N>0:
        ANS+=SUM[(x%N)]
    return ANS

NG=0
while OK>NG+1:
    #print(OK,NG)
    mid=(OK+NG)//2

    NOW=0
    PLUS=mid

    while True:
        #print(NOW,PLUS)
        k=calc(PLUS)-calc(NOW)

        NOW=PLUS
        PLUS=PLUS+k

        if PLUS>=K or PLUS>=mid+2*N:
            break
        if PLUS==NOW:
            break

    #print(mid,NOW,PLUS)

    if PLUS>=K or PLUS>=mid+2*N:
        OK=mid
    else:
        NG=mid

print(OK)