[백준] 2921 도미노 Python

n = int(input())
a = [0,0]
summ = 0
 
while a[0] != n:
    if a[0] == a[1]:
        a[1] += 1
        a[0] = 0
        summ += (a[0]+a[1])
    a[0] +=1
    summ += (a[0] + a[1])
print(summ)