#include <stdio.h>

int main() {
    int k, s = 0;
    scanf("%d", &k);
    int step = 1, step_count = 0;
    for (int i = 1; i <= k; i++) {
        if (step_count < step) {
            s += step;
            step_count++;
        }
        if (step_count == step) {
            step++;
            step_count = 0;
        }
    }
    printf("%d\n", s);
    return 0;
}