#include <iostream>
#include <cmath>
using namespace std;

int main()
{
    int a, sum = 0;
    cin >> a;

    if( a > 0 )
    {
        for ( int i = 1; i <= a; i++)
        sum += i;
    }

    else
    {
        for (int i = 1; i <= abs(a); i++)
        sum += i;
        sum = 1 - sum;
    }

    cout << sum;
    return 0;
}

hell whats problm with mine .

#include <iostream>


using namespace std;

int main()
{
    int A=0;
    double Ans=0;
    cin>>A;
    if(A<=10000)
    {
        if(A>=1)
        Ans=A*(A+1)/2;
        if(A<0)
        Ans=(A*-1)*(A-1)/2+1;
        cout<<Ans;
    }
    return 0;
}

What about less then 10000?

WA 9?
Floating types can return answer like 123e+10. So you have to do "int Ans" before got answ.

In add:
Just use formula s(a1,an)=(a1,an)/2*n like that

       int(((1+n)/2)*(abs(1-n)+1)));

Edited by author 24.08.2012 00:47

hello, whats problm with mine .
#include <iostream>
bool flag=true;//the number more than 0
int num=0;
bool GetNum()
{
    scanf("%d",&num);
    return num>=0;
}
int GetResoults()
{
    //Sn=(a1+an)n/2
    if (flag)//>0
    {
        return (1+num)*num/2;
    }
    else//<0
    {
        return (num+1)*((-num)+2)/2;
    }
}

int main()
{
    flag=GetNum();
    printf("%d\n",GetResoults());
    return 0;
}

-10000-10000