function redeem(uint256 _amount) public {
        require(_amount > 0, "You should give a number more than zero!");
        require(balances[msg.sender] > _amount, "You don't have enough balance to redeem!");
        
        uint256 feeRatio = getRedeemFee(msg.sender);
        uint256 total = _amount;
        balances[msg.sender] =  balances[msg.sender].sub(_amount);
        uint256 fee = total.mul(feeRatio).div(100);
        uint256 left = total.sub(fee);
        
        ERC20Token erc20 = ERC20Token(position[msg.sender].contractAddr);
        if (erc20.transfer(msg.sender, left) == true) {
            balances[feeOwner].add(fee);
        }
        emit Redeem(msg.sender, left, fee);
    }