#include"iostream.h"
double S,n;
int main()
{
    cin>>n;
    if(n==0)S=1;
    if(n>0)S=(n+1)*n/2;
    if(n<0)
    {
        n=n*(-1)+2;    S=(3-n)*n/2;
    }
    cout<<S<<endl;
    return 0;
}

using namespace std;

#include<iostream>
using namespace std;
int S,n;
int main()
{
    cin>>n;
    if(n==0)S=1;
    if(n>0)S=(n+1)*n/2;
    if(n<0)
    {
        n=n*(-1)+2;    S=(3-n)*n/2;
    }
    cout<<S<<endl;
    return 0;
}

The codes above are my correct codes modified from your codes.
There are three mistakes you made:
1.#include"iostream.h" should be #include<iostream>.
2.If you want to use the "cin", you should define namespace "std".
3.S and n should be defined as "int" ,not "double".