import { readFileSync } from 'fs';

function Main(input: string) {
  const data = input.split('\n');

  const firstLine = data[0].split(' ').map(Number);
  const pockeyLength = firstLine[0];
  const oneBite = firstLine[1];

  // 一口も食べない
  if (pockeyLength <= oneBite * 2) {
    console.log(0);
    process.exit();
  }

  const maxBiteCount = Math.round(Math.round(pockeyLength / oneBite) / 2);
  console.log(maxBiteCount * oneBite);
}

Main(readFileSync('/dev/stdin', 'utf8'));