#include <stdio.h>

/// <summary>
/// 入力された数字を返す
/// </summary>
/// <returns></returns>
int ReadNum() {
	int negate = 0;
	char c = getchar();
	int num = 0;
	int numCnt = 0;
	while (c != '\n') {
		if (c == '-') {
			negate = 1;
		}
		else {
			num = num * 10 + c - '0';
		}

		c = getchar();
	}
	if (negate == 1) {
		num *= -1;
	}
	return num;
}


int main()
{
	double n = ReadNum();
	double sum = 0;
	for (int i = 1;i <= 6;i++) {
		sum += (i * n) / 6;
	}
	printf("%.16g\n",sum);
}