杭電ACM 1001

Sum Problem
Time Limit: 1000/500 MS (Java/Others)    Memory Limit: 65536/32768 K (Java/Others) Total Submission(s): 406156    Accepted Submission(s): 102002
Problem Description
Hey, welcome to HDOJ(Hangzhou Dianzi University Online Judge).
In this problem, your task is to calculate SUM(n) = 1 + 2 + 3 + ... + n.
 
Input
The input will consist of a series of integers n, one integer per line.
 
Output
For each case, output SUM(n) in one line, followed by a blank line. You may assume the result will be in the range of 32-bit signed integer.
 
Sample Input

   
   
   
   

    
    
    
    1
100
   
   
   
   

 
Sample Output

   
   
   
   

    
    
    
    1


    
    
    
    
5050
    
    
    
    
    
    
    
    
#include<iostream>
using namespace std;

int main(){
    int a;
    while(cin>>a){
        if(0==a%2)///    
          cout<<(a/2*(a+1))<<endl<<endl;
        else
          cout<<(a+1)/2*a<<endl<<endl;
    }
    return 0;
}