program sample
    implicit none
  
    integer(8) :: i,j,m,n
    real(8)::a,b
    integer(8),allocatable :: x(:),y(:)
  
    read(*,*) n ,m

    read(*,*) i,j  
    write(*,*) (n-i)*(m-j)
  
    stop
end program sample
  

