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

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

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

flag=0
if SUM[-1]>=N:
    flag=1

DP=[0]*(N+1)

for i in range(1,N+1):
    NOW=DP[i-1]
    PLUS=NOW+1

    while True:
        k=calc(PLUS)-calc(NOW)

        NOW=PLUS
        PLUS=PLUS+k

        if PLUS==NOW:
            break

        if flag==1 and PLUS>i+N:
            break

    DP[i]=PLUS

    if DP[i]>=K:
        print(i)
        exit()
    if flag==1 and DP[i]>=i+N:
        print(i)
        exit()

for i in range(N+1):
    if DP[i]>=N:
        ni,nx=i,DP[i]
        break

for i in range(N+1):
    if DP[i]==nx%N:
        bi=i
        break

#print(bi,ni,nx)

rep=(K-nx)//N
ANS=ni+rep*(ni-bi)

rest=(K-nx)%N

#print(ANS,rest)

if rest==0:
    print(ANS)
    exit()

NOW=bi+1

for i in range(N+1):
    PLUS=NOW+1
    ANS+=1

    while True:
        k=calc(PLUS)-calc(NOW)

        NOW=PLUS
        PLUS=PLUS+k

        if PLUS==NOW:
            break

        if flag==1 and PLUS>i+N:
            break

    #print("!",PLUS)

    if PLUS>=rest:
        print(ANS)
        exit()
    NOW=PLUS