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ERC20.sol

pragma solidity >=0.5.0 <0.6.0;
import "https://github.com/OpenZeppelin/openzeppelin-solidity/blob/master/contracts/math/SafeMath.sol";

contract ERC20 {
    using SafeMath for uint256;
    
    uint256 private _totalSupply;
    mapping(address => uint256) private _balances;
    
    event Transfer(address indexed from, address indexed to, uint256 amount);
    
    constructor(uint256 amount) public {
        _totalSupply = amount;
        _balances[msg.sender] = amount;
    }
    
    function transfer(address to, uint256 amount) external returns (bool) {
        _balances[msg.sender] = _balances[msg.sender].sub(amount); // Remove the amount from msg.sender balance
        _balances[to] = _balances[to].add(amount);// Add the amount to "to"
        emit Transfer(msg.sender, to, amount);
        return true;
    }
        
    function totalSupply() external view returns (uint256) {
        return _totalSupply;
    }

    function balanceOf(address who) external view returns (uint256) {
        return _balances[who];
    }

}