#include<stdio.h>
#include<math.h>

int main(){
int n;
scanf("%d",&n);

if(n==0){printf("%d",1);}
else if(n>0){printf("%d",(n*(n+1))/2);}
else{
        n=fabs(n);
        printf("%d",-1*(((n*(n+1))/2)-1));
    }

return 0;
}

Edited by author 30.11.2019 13:52