#include <iostream>
#include <cstdio>
#include <sstream>
using namespace std;
int main()
{
int n;
scanf("%d",&n);
stringstream sstream;
double y;
int cnt = 0;
for(int i = 1 ; i < n ; i++)
{
sstream.clear();
int x = i * i;
x = x % n;
sstream << x;
sstream >> y;
if(y < 1.0 * n / 2) cnt++;
}
printf("%d\n",cnt);
return 0;
}