eval'N,K,*A='+$<.read.split*?,
r=0
N.times{(N-K).times{|i|A[i]>A[j=i+K]&&(r+=1;A[i],A[j]=A[j],A[i])}}
p (1...N).all?{|i|A[i-1]<A[i]}?r:-1