'''problem statement
0,0,2,1,4,2,6,3,8,4,10,5,12,6,14,7,16,8.........
testcase1
input:11
output:10
testcase2
input:28
output:13'''
................................
n=int(input())
if n%2==0:
  return n-1
else:
  return (n//2)-1