#include<bits/stdc++.h>
using namespace std;
int main(){
    int n;
    float c;
    scanf("%d",&n);
    c=(n-2)*180.0/n;
    printf("%.1f",c);       
}
/**************************************************************
	Problem: 1317
	User: xuezihong1
	Language: C++
	Result: Accepted
	Time:11 ms
	Memory:2076 kb
****************************************************************/