#include<iostream>
#include<stdio.h>
#include<algorithm>
#include<vector>
#include<string.h>
#include<math.h>
#include<map>
#include<iomanip>
#include<queue>

const long long INF = 1e6+7;
const long long MOD = 1e9+7;
const double PI=acos(-1);

using namespace std;

int main(){
	double p;
	cin >> p;
	double ans=0;
	double temp=1;
	long long TRY=0;
	while(1){
		TRY++;
		temp*=p;
		ans+=temp;
		if(TRY>=INF){
			break;
		}
	}
	cout << fixed << setprecision(10) << ans << endl;
	return 0;
}