mod = 1000000007
eps = 10**-9


def main():
    import sys
    from math import acos, sqrt
    input = sys.stdin.readline

    R, H, D = map(float, input().split())
    d = D / 2
    h = H * (R - d) / R

    ans = 0
    dh = h / (10**7)
    for x in range(10**7):
        r = R * (H - x * dh) / H
        if r < d:
            break
        e = sqrt(r ** 2 - d ** 2)
        t = acos(d / r)
        ans += (r*r*t - d*e) * dh
    print(ans * 2)


if __name__ == '__main__':
    main()