#include<bits/stdc++.h>
using namespace std;
int main(){
	float n,c;
	cin>>n;
	c=(n-2)*180/n;
	printf("%.1f",c); 
}

/**************************************************************
	Problem: 1317
	User: chensijia
	Language: C++
	Result: Accepted
	Time:14 ms
	Memory:2072 kb
****************************************************************/