include <stdio.h>

int main() {

int a, b = 0, c, d = 1, e = 1;
scanf("%d", &a);
while (e <= a) {
	for (int j = 1; j <= d; j++) {
		b = b + d;
		e++;
		if (e > a) {
			break;
		}
	}
	d++;
}
printf("%d", b);

}