integer*8::N,i,mod2,tmp,maxvalue,value,total=0
  integer*8,allocatable::a(:,:)
  read *,N
  allocate(a(1:2,1:N))
  read *,a
  mod2 = MOD(a(1,1),2)
  maxvalue  = a(1,1)+a(2,1)*4
  total = maxvalue

  do i=2, N
     tmp =  MOD(a(1,i),2)
     if(tmp.ne.mod2) then
        print '(a)','-1'
        return
     end if
     value = a(1,i) + 4*a(2,i)
     maxvalue = MAX(value, maxvalue)
     total = total + value
  end do

  print '(i0)', (maxvalue*N-total)/2
end program