求1-2+3-4+5-6+7-8...+99的和

def sum(n):    
 """求1-2+3-4+5-6+7-8...+99的和"""   
 S = 0   
 # 依次遍历1到n内的整数，奇数为正，偶数为负，再相加    
 for i in range(1, n):       
    S += ((-1)**(i + 1)) * i 
 return S

print("1-2+3-4+5-6+7-8...+99的和:" + str(sum(99)))