function main(input = ``) {
  let lines = input.split('\n');

  let [totalLength, unitLength] = lines[0]
    .split(' ')
    .map((val) => parseInt(val));

  totalLength / (unitLength * 2);

  if (totalLength % (unitLength * 2) === 0) {
    return (totalLength / (unitLength * 2) - 1) * unitLength;
  } else {
    return Math.floor(totalLength / (unitLength * 2)) * unitLength;
  }
}

const { readFileSync, existsSync } = require('fs');
const stdin = '/dev/stdin';

if (existsSync(stdin)) {
  const input = readFileSync(stdin, 'utf8');
  console.log(main(input));
}

module.exports = main;