program main 

 implicit none

 integer  :: n,nc,nd,ntmp
 integer, allocatable :: a(:),b(:),c(:)
 integer  :: i,k


 !1)
 open(10,file='input')
 read(*,*) n
 if (n.lt.3) then
    write(*,*) 'n must be => 3'
    stop 
 endif
 allocate(a(n))
 read(*,*) a

 !2)
 if (mod(n,2).eq.0) then
    nc=n-2
    allocate(b(nc))
    write(*,*) 'n is even'
    do i=1,n-2
       b(i)=a(i)+a(i+2)
    enddo
 else
    allocate(b(n))
    write(*,*) 'n is odd'
    do i=1,n
       b(i)=a(n+1-i)**3
    enddo
 endif   

 !3)
 if (mod(n,2).eq.0) then
    ntmp=nc
 else   
    ntmp=n
 endif   
 nd=0
 do i=1,ntmp
    if (b(i).lt.0) nd=nd+1
 enddo
 if (nd.gt.0) then
    allocate(c(nd))
    k=0
    do i=1,ntmp
       if (b(i).lt.0) then
          k=k+1
          c(k)=b(i)
       endif
    enddo 
    write(*,*) 'Sum of c elements',sum(c)
 endif

 !4)
 open(11,file='b.dat')
 if (mod(n,2).eq.0) then
    do i=1,nc
       write(11,100) i,b(i)
    enddo
 else
    do i=1,n
       write(11,100) i,b(i)
    enddo   
 endif
 if (nd.gt.0) then
    open(12,file='c.dat') 
    do i=1,nd
       write(12,100) i,c(i)
    enddo
    close(12)
 else
    write(*,*) 'No negative elements in c'
 endif

100 format(2(I6))

 deallocate(a)
 deallocate(b)
 if (nd.gt.0) deallocate(c)

 close(10)
 close(11)

 stop

end program main