#include <stdio.h>

float f(float x);

int main(void) {

    int a, b, i, n;

    printf("a: "); scanf("%d", &a);
    printf("b: "); scanf("%d", &b);
    printf("n: "); scanf("%d", &n);

    float h = (float) (b - a) / n, sigma = 0, x = a;

    for (i = 0; i < n; i++)
    {
        sigma += f((x + (x = x + h)) / 2) * h;
    }

    printf("%f\n", sigma);

    return 0;

}

float f(float x) {

    return 1 / x;

}