!
! --------------------------------
PROGRAM SumofTwoIntegers
 IMPLICIT NONE
 INTEGER::N,a
 INTRINSIC MOD
 READ(*,*)N
  IF(MOD(N,2)==0)THEN
  a=(N-2)/2
  WRITE(*,*)a
  ELSE
  a=(N-1)/2
  WRITE(*,*)a
  END IF
END PROGRAM SumofTwoIntegers