lines = [];
let reader = require('readline').createInterface({
  input: process.stdin,
  output: process.stdout
});

// 改行するごとに配列に入力値を格納
reader.on('line', function(line) {
  lines.push(line);
});

// ポッキーを食べるぜ
reader.on('close', function() {
  provided = lines[0].split(' ');
  const L = provided[0];
  const K = provided[1];
  let count;

  if (L % (2 * K) == 0) {
    count = L / (2 * K) - 1;
  }
  else {
    count = Math.floor(L / (2 * K));
  }
  console.log(count * K);
});