I have searched all day and still no working solution. My problem is I am working with JavaScript to get EVM blockchain data and any library I use does not work exactly.

// pair
// 0 = BMON
// 1 = BUSD

// transaction
const tx0 = 2755738843649989653057n; // amount0Out BMON
const tx1 = 113747760332474227520n; // amount1In BUSD

// reserve after transaction
const x = 466822693713887341798087n; // reserve0 BMON
const y = 19334468154310748213608n; // reserve1 BUSD
const k = x * y;
console.log(k)

const newX = x + tx0;
const newY = y - tx1;
const newK = newX * newY;
console.log(newK)

in Uniswap a price is based off the reserves. and I get data from a Swap event what and how much goes in and out.

Just for a test to see if my calculations are 100% accurate I made the above small script.

I get the reserves data after the transaction, but I need to know the price before the transaction. I subtract what's going in and add what's going out to make a new reserve. the constant of the reserve has to be always the same. my code knows the reserve after the transaction and the reserve before the transaction when I did the add/subtract but somehow the constant is never the same as it should be.

I tried bigInt, BigNumber.js, normal numbers etc. Nothing is working. and I need it to work 100% accurate.

Is there someone who can help me please?