n,m,q=gets.split.map(&:to_i)
m+=1
a=gets.split.map(&:to_i)
if a.max>=m
  puts 1
  exit
end
l=[]
mq=0
nq=0
ans=1/0.0
a.each do |aa|
  if aa%q==0
    mq=aa if mq<aa
  else
    nq=aa if nq<aa
  end
end
c=0
if mq>0&&mq>nq
  m=(m-1)/mq+1
  c=1
end
if m>1&&nq==0
  puts "-1"
  exit
end
x=1
while x<m
  x*=nq
  c+=1
end
puts c