PROGRAM TAX
 IMPLICIT NONE
 INTEGER::N,max,a,b,X
 INTRINSIC INT
 READ(*,*)N
 max=N
 b=0
 
 DO X=1, max
  a=int(X*1.08)
  IF(a == N)THEN
  b=X
  END IF
 END DO
 IF(b > 0)THEN
  WRITE(*,*)b
 ELSE
  WRITE(*,*)":("
 END IF
 
END PROGRAM TAX
