n,k = list(map(int,input().split()))
s=input()

x=0
cnt=0
C = [0]
for i,c in enumerate(s):
        if x:
            x-=1
        else:
            cnt+=1
        x+=int(c)
        C.append(cnt)
if x==0:
    cnt*=k//len(s)
    k%=len(s)
    cnt+=C[k]
elif x<cnt:
    a = [i for i in range(cnt-x+1)][::-x]
    cnt += sum(a)
    
print(cnt)