#include<stdio.h>
int main()
{
long int n, c;
while(scanf("%ld", &n) && !(n<0))
{
if(n%2!=0)
{
c = n*((n+1)/2)+1;
printf("%ld\n", c);
}
else
{
c = n*(n/2)+((n/2)+1);
printf("%ld\n", c);
}
}
return 0;
}
int main()
{
long int n, c;
while(scanf("%ld", &n) && !(n<0))
{
if(n%2!=0)
{
c = n*((n+1)/2)+1;
printf("%ld\n", c);
}
else
{
c = n*(n/2)+((n/2)+1);
printf("%ld\n", c);
}
}
return 0;
}
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