Solidity 101

pragma solidity ^0.5.1;

contract count {
    
    uint counter;
    address paid;
    
     struct counting {
        address counter; 
        string message; 
        uint value; 
    }

    counting[] public numbers; 
    mapping(address => uint) amount; 
    

    
    constructor () public {
        counter = 0; 
    }
    
    function increment(uint _counter, string calldata message) payable external {
        counter += _counter;
        numbers.push(counting(msg.sender, message, msg.value));
        amount[msg.sender] = msg.value;
        
    }
    
    function decrement() external {
        //require(counter > 0); 
        counter --;
    }
    
    function get() external view returns(uint){
        return counter;
    }
    
    function () payable external {
        counter += msg.value;
        paid = msg.sender;
        
        
    }
    
    
    function getMoney() external view returns (uint){
        return address(this).balance;
    } 
    
    function getPaid() external view returns (address){
        return paid; 
    }
    function getNumbers(uint index) external view returns (address, string memory, uint){
        counting memory object =  numbers[index];
        return (object.counter, object.message, object.value);
        
    }
    function getAmount(address key) external view returns (uint){
        return amount[key];
    }
    
}