编写程序，输入n的值，计算Sn = 1 - 3 + 5 - 7 + 9 – 11…，n为此数列的项数（如n为3，输出前3项）用python写

答案：
def calculateSn(n):
sn = 0
for i in range(1, n+1, 2):
if i % 4 == 1:
sn += i
else:
sn -= i
return snprint(calculateSn(n))