program main
   implicit none
   integer(8) ::n,summ=0
   integer(8) i
   integer(8),allocatable,dimension(:)::a,b,c
   read(*,*)n
   allocate(a(n),b(n),c(n))
   do i=1,n
      read(*,*)a(i),b(i)
      if(i>1)then
         if(mod(abs(a(i)-a(i-1)),2)/=0)then
            write(*,*)-1
            goto 10
         endif
      endif
      c(i)=a(i)+4*b(i)
   enddo
   do i=1,n
      summ=summ+(maxval(c)-c(i))
   enddo
   write(*,*)summ/2
10 continue   
end program main