Lagrange Interpolation using Fortran

input.txt

1.0 181.0
2.0 155.0
3.0 161.5
4.0 183.0
6.0 214.1
10.0 319.0

lagrange.f90

program lagrange
    implicit none
    integer, parameter :: n = 6
    integer :: i, j
    real :: x(n), y(n), output, xp, p

    open(unit = 2, file = 'input.txt')

    do i = 1, n
        read(2,*)x(i), y(i)
    end do

    do i = 1, n
        write(*,*)x(i), y(i)
    end do

    output = 0; xp = 7!Approximation point
    do i = 1, n
        p = 1
        do j = 1, n
            if(i == j) then
                p = p * 1
                else
                    p = p * (xp-x(j)) / (x(i)-x(j))
            end if
        end do
        output = output + p * y(i)
    end do

    write(*,'(2/, A24, F10.4, 2/)')"Approximate result : ", output

end program