1 #include <stdio.h>
 2 #include <algorithm>
 3 #include <string.h>
 4 #include <iostream>
 5 #include <vector>
 6 #include <map>
 7 #include <queue>
 8 #include <set>
 9 #include <vector>
10 #include <string>
11 #include <sstream>
12 using namespace std;
13 
14 main()
15 {
16     int n;
17     while(scanf("%d",&n)==1&&n)
18     {
19         printf("%d\n",(n-1)>>1);
20     }
21 }