function Main(input) {
  // const data = input.split("\n");
  let data = input.split(" ");
  let L = data[0];
  let K = data[1];

  let count = 0;

  if ( L/K <= 2 ) {
    count = 0;
  } else {  
    if ( L%K === 0 ) {
      if ( (L/K)%2 === 0 ) {
        count = ((L/K)-2)/2;
      } else {
        count = ((L/K)-1)/2;
      }
    } else { 
      if ( Math.floor(L/K)%2 === 0 ) {
        count = (Math.floor(L/K))/2;
      } else {
        count = (Math.floor(L/K)-1)/2;
      }    
    }
  }
  console.log(count * K);
}
Main(require("fs").readFileSync("/dev/stdin", "utf8"));