554 Пицца - Динамика

#include <stdio.h>

int main( )
{
    FILE *in = fopen("input.txt", "rt");
    FILE *out = fopen("output.txt", "wt");

    if (in && out)
    {
        int n;
        fscanf(in, "%d", &n);

        int a = 2;

        for (int i = 2; i <= n; ++i)
        {
            a += i;
        }

        fprintf(out,"%d\n", a);
    }

    return 0;
}