"use strict";

function _slicedToArray(arr, i) { return _arrayWithHoles(arr) || _iterableToArrayLimit(arr, i) || _nonIterableRest(); }

function _nonIterableRest() { throw new TypeError("Invalid attempt to destructure non-iterable instance"); }

function _iterableToArrayLimit(arr, i) { var _arr = []; var _n = true; var _d = false; var _e = undefined; try { for (var _i = arr[Symbol.iterator](), _s; !(_n = (_s = _i.next()).done); _n = true) { _arr.push(_s.value); if (i && _arr.length === i) break; } } catch (err) { _d = true; _e = err; } finally { try { if (!_n && _i["return"] != null) _i["return"](); } finally { if (_d) throw _e; } } return _arr; }

function _arrayWithHoles(arr) { if (Array.isArray(arr)) return arr; }

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

  var _splitNum = splitNum(lines[0]),
      _splitNum2 = _slicedToArray(_splitNum, 2),
      N = _splitNum2[0],
      K = _splitNum2[1];

  var as = splitNum(lines[1]).slice(0, N);

  if (as.indexOf(1) >= as.length / 2) {
    as.reverse();
  }

  var one = as.indexOf(1);
  var result = 0;
  var oner = one; // for left

  if (one >= K - 1) {
    result += Math.ceil(one / (K - 1));
  } else if (one > 0) {
    result++;
    oner = K - 1;
  } // for right


  result += Math.ceil((N - oner - 1) / (K - 1));
  console.log(result);
}

main(require('fs').readFileSync('/dev/stdin', 'utf8'));

function splitNum(line) {
  return line.split(/\s+/).map(function (n) {
    return parseInt(n);
  });
}
