if (_totalSupply == 0) {
    address migrator = IUniswapV2Factory(factory).migrator();
    if (msg.sender == migrator) {
        liquidity = IMigrator(migrator).desiredLiquidity();
        require(liquidity > 0 && liquidity != uint256(-1), "Bad desired liquidity");
    } else {
        require(migrator == address(0), "Must not have migrator");
        liquidity = Math.sqrt(amount0.mul(amount1)).sub(MINIMUM_LIQUIDITY);
        _mint(address(0), MINIMUM_LIQUIDITY); // permanently lock the first MINIMUM_LIQUIDITY tokens
    }
} else {
    liquidity = Math.min(amount0.mul(_totalSupply) / _reserve0, amount1.mul(_totalSupply) / _reserve1);
}
function migrate(IUniswapV2Pair orig) public returns (IUniswapV2Pair) {
    require(msg.sender == chef, "not from master chef");
    require(block.number >= notBeforeBlock, "too early to migrate");
    require(orig.factory() == oldFactory, "not from old factory");
    address token0 = orig.token0();
    address token1 = orig.token1();
    IUniswapV2Pair pair = IUniswapV2Pair(factory.getPair(token0, token1));
    if (pair == IUniswapV2Pair(address(0))) {
        pair = IUniswapV2Pair(factory.createPair(token0, token1));
    }
    uint256 lp = orig.balanceOf(msg.sender);
    if (lp == 0) return pair;
    desiredLiquidity = lp;
    orig.transferFrom(msg.sender, address(orig), lp);
    orig.burn(address(pair));
    pair.mint(msg.sender);
    desiredLiquidity = uint256(-1);
    return pair;
}
