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

int main(){
int n,f,i;
int a[10001];
a[1]=2;
for(i=2;i<=10000;i++){
a[i]=a[i-1]+i;
}
while(scanf("%d",&n)!=EOF){
printf("%d\n",a[n]);
}

return 0;
}