function executeSell() private {
    uint256 tokensToSell;
    require(msg.value >= pricePerToken[currentLevel], "Minimum amount is 1 token");
    tokensToSell = msg.value.div(pricePerToken[currentLevel]);
    tkn.mintTo(msg.sender, tokensToSell);
    tokensSold = tokensSold.add(tokensToSell);
    walletAddress.transfer(msg.value);
  }