#input of int(split by space)
def get_i()
  return gets.chomp.split(" ").map(&:to_i)
end
#input of float(split by space)
def get_f()
  return gets.chomp.split(" ").map(&:to_f)
end
#input of string(split by space)
def get()
  return gets.chomp.split(" ")
end
#input of string(split per one character)
def get_nsp()
  return gets.chomp.split("")
end
#yes or no decision
def yn_judge(bool,y="Yes",n="No")
  return bool ? y : n 
end
#create of array
def array(size1,init=nil,size2=-1)
  if size2==-1
    return Array.new(size1){init}
  else
    return Array.new(size2){Array.new(size1){init}}
  end
end

def max(a,b)
    return a>b ? a : b
end

def min(a,b)
    return a>b ? b : a
end

INF=Float::INFINITY

n,m=get_i
s=get_nsp
t=get_nsp
if n>m
    n,m=m,n
    s,t=t,s
end
unless (m%n).zero?
    if s[0]==t[0]
        puts n.lcm(m)
    else
        puts -1
    end
else
    x=m/n
    flag=true
    n.times do|i|
      if s[i]!=t[i*x]
          flag=false
          break
      end 
    end
    if flag
        puts n.lcd(m)
    else
        puts -1
    end
end