#include<bits/stdc++.h>
using namespace std;
int main(){

	double n;
	cin>>n;
	cout<<fixed<<setprecision(1)<<((n-2)*180)/n<<endl;


	return 0;
}

/**************************************************************
	Problem: 1317
	User: admin
	Language: C++
	Result: Accepted
	Time:12 ms
	Memory:2072 kb
****************************************************************/