PROGRAM main
    IMPLICIT NONE
    REAL :: X

    CALL diff (X, 1.23, 4.56)
    PRINT *, X
    CALL diff (X, -7.89, 3.42)
    PRINT *, X
    CALL diff (X, 4.29, -0.98)
    PRINT *, X

CONTAINS

    SUBROUTINE diff (P, Q, R)
        IMPLICIT NONE
        REAL, INTENT(OUT) :: P
        REAL, INTENT(IN) :: Q, R
        P = Q-R
    END SUBROUTINE diff

END PROGRAM main