PROGRAM Math_Magic
        IMPLICIT NONE
        INTEGER :: num, tnum
outer:  DO
            PRINT *, "Type in your number (0 terminates)"
            READ *, num
            IF (num .LE. 0) EXIT
inner:      DO
                tnum = num/2
                IF (2*tnum .EQ. num) THEN    ! num is even
                    num = tnum
                ELSE    ! num is odd
                    num = 3*num+1
                END IF
                PRINT *, num
                IF (num == 1) THEN
                    PRINT *, "Sequence finishes nicely"
                    EXIT
                ELSE IF (num == 13) THEN
                    PRINT *, "Yoiks, extreme bad luck encountered"
                    EXIT outer
                END IF
            END DO inner
        END DO outer
    END PROGRAM Math_Magic