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

  let N = Math.floor(L/K);
  let countYu = Math.floor(N/2);

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