def s(N, z, M, o, K):
    if N == 0 or M == 0:
        if K == 0:
            return N*z+M*o
        else:
            return -1
    if K == 0:
        return -1
    if K > 2*M:
        return -1
    if K >= 2*N:
        return -1
    if K % 2 == 0:
        return (z+o)*(K//2) + o * (M-K//2) + z * (N-K//2)
    else:
        return z * (N-K//2) + (o+z)*(K//2) + o * (M-K//2)


def main():
    N, M, K = map(int, input().split())
    if N >= M:
        R = s(N, '0', M, '1', K)
    else:
        R = s(M, '1', N, '0', K)
    print(R)


if __name__ == '__main__':
    main()