// SPDX-License-Identifier: MIT pragma solidity ^0.8.0;

interface IGmpReceiver { function onGmpReceived(bytes32 id, uint128 network, bytes32 source, bytes calldata payload) external payable returns (bytes32); }

contract Counter is IGmpReceiver { address private immutable _gateway; uint256 public number;

// example gw 0x7702eD777B5d6259483baAD0FE8b9083eF937E2A
constructor(address gateway) {
    _gateway = gateway;
}

function onGmpReceived(bytes32, uint128, bytes32, bytes calldata) external payable returns (bytes32) {
    require(msg.sender == _gateway, "unauthorized");
    number++;
    return bytes32(number);
}

}