n,k=input().strip().split(" ")
n,k=(int(n),int(k))
a=list(map(int,input().split(" ")))
i,j,trans,flag,loc=0,0,0,0,0
while(i<n):
    trans+=1
    j=i+k-1
    if(j>n):
        j=n-1
    while (loc<=j<n and a[j]==0):
        j-=1
    if(j<loc):
        print("-1 ")
        flag=1
        break
    else:
        loc=j+1
        j+=k
        i=j
if(flag==0):      
    print(trans)