contract SafeVault {
  mapping(address => uint256) public balances;
  
  function deposit() external payable {
    balances[msg.sender] += msg.value;
    emit Deposit(msg.sender, msg.value);
  }
}

function mint(address to, string memory tokenURI) 
  public onlyOwner returns (uint256) {
  _tokenIds.increment();
  uint256 newItemId = _tokenIds.current();
  _mint(to, newItemId);
  _setTokenURI(newItemId, tokenURI);
  return newItemId;
}

function calculateReward(address user) 
  public view returns (uint256) {
  uint256 stakingPeriod = block.timestamp - 
    stakingStartTime[user];
  return (stakedAmount[user] * APY * stakingPeriod) 
    / (365 days * 100);
}