# include <stdio.h>

int main(){

int a;

while(scanf("%d", &a)!= EOF){

int soma = 0;

    if((a<=10000) && (a>=-10000)){

        if(a<0){
            while(a<=1){

                soma+= a;
                a++;
            }
        }
        else{
            while(a>=1){
                soma+= a;
                a--;
            }
        }
    printf("%d\n",soma);
    }
}

return (0);
}