#include<stdio.h>
#include<stdlib.h>
#include<stdint.h>
#include<inttypes.h>
#include<math.h>

typedef int32_t i32;
typedef int64_t i64;

double PARAM = 0.0;

double func (double r) {
    if (2 * r < PARAM) {
        return 0;
    }
    double p = acos (PARAM / (2 * r)) * 2;
    return r * r * (p - sin(p));
}

double calc (void) {
    const i32 n = 1 << 26;
    double f = (double) 1 / n;
    double sum = 0;
    for (i32 i = n - 1; i >= 0; --i) {
        double r = (n - i) * f;
        sum += func(r);
    }
    return sum * f;
}

void run (void) {
    double r, h, d;
    scanf ("%lf%lf%lf", &r, &h, &d);
    PARAM = d / r;
    double ans = r * r * h * calc();
    printf ("%7f", ans);
}

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