func (s *Service) GetContractBalance(ctx context.Context, chainID string, blockID string, contractID string) (*big.Int, error) {
    u := "/chains/" + chainID + "/blocks/" + blockID + "/context/contracts/" + contractID + "/balance"
    req, err := s.Client.NewRequest(ctx, http.MethodGet, u, nil)
    if err != nil {
        return nil, err