a.f90

PROGRAM Calibration
  IMPLICIT NONE

  INTEGER :: total_sum, first_digit, last_digit, calibration_value
  CHARACTER(100), DIMENSION(:), ALLOCATABLE :: calibration_document
  INTEGER :: i, len_line, char_value, status

  ! Initialize total sum
  total_sum = 0

  ! Read calibration document from input.txt
  OPEN(UNIT=10, FILE='input.txt', STATUS='OLD', ACTION='READ', IOSTAT=status)
  ALLOCATE(calibration_document(1000))
  i = 1
  DO WHILE (i <= 1000 .AND. status == 0)
    READ(10, '(A100)', IOSTAT=status) calibration_document(i)
    i = i + 1
  ENDDO
  CLOSE(UNIT=10)

  ! Calculate the sum of calibration values
  DO i = 1, SIZE(calibration_document)
    len_line = LEN_TRIM(calibration_document(i))
    IF (len_line > 0) THEN
      ! Filter out non-digit characters and take the first and last digits
      last_digit = -1
      DO WHILE (len_line > 0 .AND. last_digit < 0)
        char_value = IACHAR(calibration_document(i)(len_line:len_line))
        IF (char_value >= IACHAR('0') .AND. char_value <= IACHAR('9')) THEN
          last_digit = char_value - IACHAR('0')
        ENDIF
        len_line = len_line - 1
      ENDDO

      first_digit = -1
      len_line = 1
      DO WHILE (len_line <= len_trim(calibration_document(i)) .AND. first_digit < 0)
        char_value = IACHAR(calibration_document(i)(len_line:len_line))
        IF (char_value >= IACHAR('0') .AND. char_value <= IACHAR('9')) THEN
          first_digit = char_value - IACHAR('0')
        ENDIF
        len_line = len_line + 1
      ENDDO

      ! Combine the digits to form a two-digit number
      calibration_value = first_digit * 10 + last_digit

      ! Add the calibration value to the total sum
      total_sum = total_sum + calibration_value
    ENDIF
  ENDDO

  ! Print the result
  PRINT *, 'Sum of calibration values:', total_sum

  ! Deallocate memory
  DEALLOCATE(calibration_document)

END PROGRAM Calibration