function sockMerchant(n, ar) {
    // Write your code here
    let duplicate = [];
    let sar = ar.sort((a, b) => a- b);
    for(let i=0; i<n; i++){
        if(sar[i] === sar[i+1]){
            duplicate.push(sar[i]);
            ++i;
        }
    }
    return duplicate.length;
}