A simple counter written in Solidity, for Ethereum.

counter.sol

pragma solidity ^0.4.0;
contract Counter {
    int private count = 0;
    function incrementCounter() public {
        count += 1;
    }
    function decrementCounter() public {
        count -= 1;
    }
    function getCount() public constant returns (int) {
        return count;
    }
}

for the new version of solidity.

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.4;

contract TestCounter {
    int private count = 0;
    function incrementCounter() public {
        count += 1;
    }
    function decrementCounter() public {
        count -= 1;
    }

    function getCount() public view returns (int) {
        return count;
    }
}