# 读取输入的奇数 n
n = int(input())

# 计算从 1 到 n 的奇数和
total = 0
for i in range(1, n + 1, 2):
    total += i

# 输出结果
print(total)
/**************************************************************
	Problem: 1003
	User: a99344
	Language: Python
	Result: Accepted
	Time:79 ms
	Memory:34244 kb
****************************************************************/