#include<bits/stdc++.h>
using namespace std;
int main(){
	int a;
	float b;
	cin>>a;
	b=(a-2)*180.0/a;
	printf("%.1f",b);
	return 0;
}

/**************************************************************
	Problem: 1317
	User: chenjunlin
	Language: C++
	Result: Accepted
	Time:13 ms
	Memory:2072 kb
****************************************************************/