function findShortestPath(N, bricks) {
  let visited = new Set();
  let queue = [[0, 0]]; 

  while (queue.length > 0) {
    let [position, moves] = queue.shift(); 

    if (position === N - 1) {
      return moves;
    }

    visited.add(position); 
    if (position + 1 < N && bricks[position] === bricks[position + 1] && !visited.has(position + 1)) {
      queue.push([position + 1, moves + 1]); 
    }

    if (position - 1 >= 0 && bricks[position] === bricks[position - 1] && !visited.has(position - 1)) {
      queue.push([position - 1, moves + 1]); 
    }

    for (let i = 0; i < N; i++) {
      if (bricks[i] === bricks[position] && !visited.has(i)) {
        queue.push([i, moves + 1]); 
      }
    }
  }
  return -1; 
}

process.stdin.on('data', data => {
  inputArr.push(data.toString().trim());
  if (++count === 3) {
    console.log(divideMutantsIntoTeams(inputArr));
    process.exit(0);
  }
});

function divideMutantsIntoTeams(inputArr){
  let N = parseInt(inputArr[0], 10);
  let K = parseInt(inputArr[1], 10);
  let mutants = inputArr[2].split(' ').map(Number);


  if (N < K || new Set(mutants).size === 1 || N % K !== 0) {
    return -1;
  }


  mutants.sort((a, b) => a - b);
  let minNonCompatibilityScore = Infinity;

  for (let i = 0; i <= N - K; i++) {
    let diff = mutants[i + K - 1] - mutants[i];
    if (diff < minNonCompatibilityScore) {
      minNonCompatibilityScore = diff;
    }
  }

  return minNonCompatibilityScore;
}

var stdin process.openStdin();
stdin.addListener("data", function(d) {

// note: d is an object, and when converted to a string it will
// end with a linefeed. so we (rather crudely) account for that
// with toString() and then trim()
console.log(d.toString().trim());

});