Find the 15th term of the series?0,0,7,6,14,12,21,18, 28 code example

Example: Find the 15th term of the series?0,0,7,6,14,12,21,18, 28

n = int(input())
ll = [0,0]
for i in range(n//2):
    ll.append(ll[-2] + 7)
    ll.append(ll[-2] + 6)
print(ll[n])