sum=n/k; 
你确定k能整除n吗.比如n=5;k=2,就不能这样除.
不能确定整除的时候,不要用除法.
如果你先全部乘的话,又会越过int取值范围.不要硬搞了,想一想处理方法吧.
Warning: Don't underestimate the problem. The result will fit into an 
integer - but if all intermediate results arising during the computation 
will also fit into an integer depends on your algorithm. The test cases will go to the limit.