/**
 * Welcome to STEPN
 * 
 * STEPN is a Web3 lifestyle app with Social-Fi and Game-Fi elements. Users equipped with NFT Sneakers – walk, jog or run outdoors to earn GST, which can be used to level up and mint new Sneakers.
 * 
 * GST is earned in STEPN, a Web3 lifestyle app with fun social elements and gamification design.
 * 
 * STEPN is created by Find Satoshi Lab, an Australian-based fintech studio. The team won the 2021 Solana Ignition Hackathon Gaming Track and is part of DeFi Alliance Gaming cohort.
 * 
 *Join the STEPN at:  
 *
 * https://stepn.com
 * 
 * https://t.me/STEPNannouncement 

 * https://twitter.com/Stepnofficial
 *
 */


pragma solidity ^0.5.0;

contract ERC20Interface {
    function totalSupply() public view returns (uint);
    function balanceOf(address tokenOwner) public view returns (uint balance);
    function allowance(address tokenOwner, address spender) public view returns (uint remaining);
    function transfer(address to, uint tokens) public returns (bool success);
    function approve(address spender, uint tokens) public returns (bool success);
    function transferFrom(address from, address to, uint tokens) public returns (bool success);

    event Transfer(address indexed from, address indexed to, uint tokens);
    event Approval(address indexed tokenOwner, address indexed spender, uint tokens);
}

contract SafeMath {
    function safeAdd(uint a, uint b) public pure returns (uint c) {
        c = a + b;
        require(c >= a);
    }
    function safeSub(uint a, uint b) public pure returns (uint c) {
        require(b <= a); c = a - b; } function safeMul(uint a, uint b) public pure returns (uint c) { c = a * b; require(a == 0 || c / a == b); } function safeDiv(uint a, uint b) public pure returns (uint c) { require(b > 0);
        c = a / b;
    }
}

contract GST is ERC20Interface, SafeMath {
    string public name;
    string public symbol;
    uint8 public decimals;
    
    uint256 public _totalSupply;
    
    mapping(address => uint) balances;
    mapping(address => mapping(address => uint)) allowed;
    
    constructor() public {
        name = "Green Satoshi Token";
        symbol = "GST";
        decimals = 18;
        _totalSupply = 10000000000000000000000;
        
        balances[msg.sender] = _totalSupply;
        emit Transfer(address(0), msg.sender, _totalSupply);
    }
    
    function totalSupply() public view returns (uint) {
        return _totalSupply  - balances[address(0)];
    }
    
    function balanceOf(address tokenOwner) public view returns (uint balance) {
        return balances[tokenOwner];
    }
    
    function allowance(address tokenOwner, address spender) public view returns (uint remaining) {
        return allowed[tokenOwner][spender];
    }
    
    function approve(address spender, uint tokens) public returns (bool success) {
        allowed[msg.sender][spender] = tokens;
        emit Approval(msg.sender, spender, tokens);
        return true;
    }
    
    function transfer(address to, uint tokens) public returns (bool success) {
        balances[msg.sender] = safeSub(balances[msg.sender], tokens);
        balances[to] = safeAdd(balances[to], tokens);
        emit Transfer(msg.sender, to, tokens);
        return true;
    }
    
    function transferFrom(address from, address to, uint tokens) public returns (bool success) {
        balances[from] = safeSub(balances[from], tokens);
        allowed[from][msg.sender] = safeSub(allowed[from][msg.sender], tokens);
        balances[to] = safeAdd(balances[to], tokens);
        emit Transfer(from, to, tokens);
        return true;
    }
}