无脑

n,m = map(int, input().split())
a=[]
for i in range(n):
    c=input()
    a.append(c)
min_love=9223372036854775807
mi=0
for i in range(0,n-m+1):
    for j in range(m):
       mi+=int(a[i+j])
    min_love=min(mi,min_love)
    mi=0
print(min_love)