#include <iostream>
using namespace std;
int main()
{
	float n;
	cin >> n;
	printf("%.1f",(n-2)*180/n);
	
	
}
/**************************************************************
	Problem: 1317
	User: xiaomingze
	Language: C++
	Result: Accepted
	Time:13 ms
	Memory:2072 kb
****************************************************************/