Timus Online Judge 1068. Sum

#include<bits/stdc++.h>

using namespace std;

/// Typedef
typedef long long int ll;

//Long Data type
#define scl1(a)             scanf("%lld", &a)
#define scl2(a, b)          scanf("%lld %lld", &a, &b)
#define scl3(a, b, c)       scanf("%lld %lld %lld", &a, &b, &c)
#define scl4(a, b, c, d)    scanf("%lld %lld %lld %lld", &a, &b, &c, &d)



int main(){

   ll i,j,n,m,a=0,b=0,c=0,d=0,e=0,f=0,g=0,t,ln,ck=0,dk=0, loc2=0;

   scl1(n);
   if(n>0){
   for(i=1; i<=n; i++){
     ck+=i;
   }
   cout<<ck<<endl;
   }
   else{
    for(i=1; i>=n; i--){
        ck+=i;
    }
    cout<<ck<<endl;
   }

}

Or

#include<bits/stdc++.h>

using namespace std;

/// Typedef
typedef long long int ll;

//Long Data type
#define scl1(a)             scanf("%lld", &a)
#define scl2(a, b)          scanf("%lld %lld", &a, &b)
#define scl3(a, b, c)       scanf("%lld %lld %lld", &a, &b, &c)
#define scl4(a, b, c, d)    scanf("%lld %lld %lld %lld", &a, &b, &c, &d)



int main(){

   ll i,j,n,m,a,d,t,l,k,b,c,ck=0,dk=0, loc2=0;

   scl1(n);

   if(n>1)
     m = n;
   else
    m = (n*-1)+2;
   ck = m*(n+1);
   cout<<ck/2<<endl;

}