IDENTIFICATION DIVISION.
PROGRAM-ID. A-BEGINNER.
 
DATA DIVISION.
WORKING-STORAGE SECTION.
01 INP     PIC X(100).
01 IN-DATA.
   03 DT-X1 PIC X(10).
   03 DT-X2 PIC X(10).
01 IN-LEN.
   03 DT-L1 PIC 9(2).
   03 DT-L2 PIC 9(2).
01 IN-A     PIC 9(10).
01 IN-B     PIC 9(10).
01 WK-K     PIC 9(10).
01 OUT-K    PIC Z(8)9.
 
PROCEDURE DIVISION.
MAIN-001.
  ACCEPT INP.
  UNSTRING INP DELIMITED BY ALL ' '
      INTO DT-X1 COUNT IN DT-L1
           DT-X2 COUNT IN DT-L2.
  MOVE DT-X1(1:DT-L1) TO IN-A.
  MOVE DT-X2(1:DT-L2) TO IN-B.
 
  COMPUTE WK-K = (IN-A + IN-B) / 2.
  IF (WK-K * 2 = IN-A + IN-B)
    MOVE WK-K TO OUT-K
    DISPLAY OUT-K
  ELSE
    DISPLAY 'IMPOSSIBLE'
  END-IF.

MAIN-EXIT.
  STOP RUN.