function rbuyTokens(address recipient_addr, uint256 _numberOfTokens) public payable {
        require(msg.sender==admin);
        require(msg.value == multiply(_numberOfTokens, tokenPrice));
        require(tokenContract.balanceOf(address(this)) >= _numberOfTokens);
        require(tokenContract.transfer(recipient_addr, _numberOfTokens));

        tokensSold += _numberOfTokens;

        emit Sell(msg.sender, _numberOfTokens);
    }