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


/**************************************************************
	Problem: 1317
	User: lyy002
	Language: C++
	Result: Accepted
	Time:13 ms
	Memory:2072 kb
****************************************************************/