n,m=map(int,input().split())
p=list(map(int,input().split()))
B=450
st1=[0]*B*B
st2=[0]*B
f=0
for v in p[::-1]:
  l=0
  r=v
  yl=l//B
  yr=r//B
  if yl==yr:
    f+=sum(st1[l:r+1])
  else:
    f+=sum(st1[l:yl*B+B])
    f+=sum(st2[yl+1:yr])
    f+=sum(st1[yr*B:r+1])
  st1[v]+=1
  st2[v//B]+=1
m*=(f+m-1)//m
if m%2==1 and f%2==0:
  m*=2
print(m if (m-f)%2==0 else -1)