"use strict";

function _createForOfIteratorHelper(o) { if (typeof Symbol === "undefined" || o[Symbol.iterator] == null) { if (Array.isArray(o) || (o = _unsupportedIterableToArray(o))) { var i = 0; var F = function F() {}; return { s: F, n: function n() { if (i >= o.length) return { done: true }; return { done: false, value: o[i++] }; }, e: function e(_e2) { throw _e2; }, f: F }; } throw new TypeError("Invalid attempt to iterate non-iterable instance.\nIn order to be iterable, non-array objects must have a [Symbol.iterator]() method."); } var it, normalCompletion = true, didErr = false, err; return { s: function s() { it = o[Symbol.iterator](); }, n: function n() { var step = it.next(); normalCompletion = step.done; return step; }, e: function e(_e3) { didErr = true; err = _e3; }, f: function f() { try { if (!normalCompletion && it.return != null) it.return(); } finally { if (didErr) throw err; } } }; }

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

function _nonIterableRest() { throw new TypeError("Invalid attempt to destructure non-iterable instance.\nIn order to be iterable, non-array objects must have a [Symbol.iterator]() method."); }

function _unsupportedIterableToArray(o, minLen) { if (!o) return; if (typeof o === "string") return _arrayLikeToArray(o, minLen); var n = Object.prototype.toString.call(o).slice(8, -1); if (n === "Object" && o.constructor) n = o.constructor.name; if (n === "Map" || n === "Set") return Array.from(n); if (n === "Arguments" || /^(?:Ui|I)nt(?:8|16|32)(?:Clamped)?Array$/.test(n)) return _arrayLikeToArray(o, minLen); }

function _arrayLikeToArray(arr, len) { if (len == null || len > arr.length) len = arr.length; for (var i = 0, arr2 = new Array(len); i < len; i++) { arr2[i] = arr[i]; } return arr2; }

function _iterableToArrayLimit(arr, i) { if (typeof Symbol === "undefined" || !(Symbol.iterator in Object(arr))) return; 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; }

var mod = 998244353;

var processData = function processData(lines) {
  var _lines$0$split$map = lines[0].split(' ').map(function (x) {
    return +x;
  }),
      _lines$0$split$map2 = _slicedToArray(_lines$0$split$map, 2),
      n = _lines$0$split$map2[0],
      k = _lines$0$split$map2[1];

  var h = lines[1].split(' ').map(function (x) {
    return +x;
  });
  var min = n;
  var max = 0;

  var _iterator = _createForOfIteratorHelper(h),
      _step;

  try {
    for (_iterator.s(); !(_step = _iterator.n()).done;) {
      var _m = _step.value;

      if (_m > max) {
        max = _m;
      }

      if (_m < min) {
        min = _m;
      }
    }
  } catch (err) {
    _iterator.e(err);
  } finally {
    _iterator.f();
  }

  var diff = max - min;
  var diffChanged = true;
  var mark = h.length - 1;

  while (diffChanged && mark > 0) {
    var m = h[mark] - n;
    var possibleMin = m;
    var possibleMax = h[mark - 1];

    if (possibleMax - possibleMin < diff) {
      diffChanged = true;
      diff = possibleMax - possibleMin;
    }

    mark--;
  }

  console.log(diff);
};

var i = '';
process.stdin.on('data', function (c) {
  return i += c;
});
process.stdin.on('end', function () {
  var _require = require('os'),
      EOL = _require.EOL;

  var lines = i.split(EOL);
  /*your input text, split by lines*/

  processData(lines);
});
