#include <iostream>
#include <stdio.h>
using namespace std;
int main(int argc, char* argv[])
{
	int N;
	cin>>N;
	double x=3.5*N;
	if (x==(int)x){
		printf("%ld\n",(int)x);
	}else{
		printf("%.3lf\n",x);
	}
	return 0;
}