#include<bits/stdc++.h>
using namespace std;
int n;
int main(){
	cin>>n;
	printf("%.1lf",((n-2.0)*180)/(n*1.0));
	return 0;
}
/**************************************************************
	Problem: 1317
	User: zhangziang
	Language: C++
	Result: Accepted
	Time:13 ms
	Memory:2072 kb
****************************************************************/