uva 10079 - Pizza Cutting

#include   
long long sum;  
int n;  
int main () {  
    while (scanf("%d", &n), n >





= 0) {  
        sum = 1;  
        for (long long i = 1; i <= n; i++)  
            sum += i;  
        printf("%lld\n", sum);  
    }  
    return 0;  
}