#include <stdio.h>
#include <math.h>

double sum_arithmetic(double x0, double r, int n)
{
 return n == 0 ? 0 : x0 + sum_arithmetic(x0 + r, r, n-1);
}

void test_sum_arithmetic(void)
{
    double x0;
    double r;
    int n;
    while (scanf("%f%f%d", &x0, &r, &n) !=EOF)
    {
        double z = sum_arithmetic(x0, r, n);
        printf("%f\n", z);
    }
}

int main(void)
{
    test_sum_arithmetic();
    return 0;
}