import std.conv, std.functional, std.range, std.stdio, std.string;
import std.algorithm, std.array, std.bigint, std.bitmanip, std.complex, std.container, std.math, std.mathspecial, std.numeric, std.regex, std.typecons;
import core.bitop;

class EOFException : Throwable { this() { super("EOF"); } }
string[] tokens;
string readToken() { for (; tokens.empty; ) { if (stdin.eof) { throw new EOFException; } tokens = readln.split; } auto token = tokens.front; tokens.popFront; return token; }
int readInt() { return readToken.to!int; }
long readLong() { return readToken.to!long; }

string COLOR(string s = "") { return "\x1b[" ~ s ~ "m"; }

bool chmin(T)(ref T t, in T f) { if (t > f) { t = f; return true; } else { return false; } }
bool chmax(T)(ref T t, in T f) { if (t < f) { t = f; return true; } else { return false; } }

int binarySearch(alias pred, T)(in T[] as) { int lo = -1, hi = cast(int)(as.length); for (; lo + 1 < hi; ) { const mid = (lo + hi) >> 1; (unaryFun!pred(as[mid]) ? hi : lo) = mid; } return hi; }
int lowerBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a >= val)); }
int upperBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a > val)); }




void main() {
  try {
    for (; ; ) {
      const L = readInt;
      const N = readInt + 1;
      auto X = new int[N + 1];
      foreach (i; 1 .. N) {
        X[i] = readInt;
      }
      X[0] = 0;
      X[N] = L;
      
      auto dp = new bool[][](2, L + 1);
      dp[0][0] = true;
      foreach (i; 0 .. N) {
        const d = X[i + 1] - X[i];
        foreach_reverse (y; 0 .. L + 1) {
          dp[1][y] = (y >= d && dp[1][y - d]);
        }
        if (i + 1 < N) {
          foreach (y; 0 .. L + 1) {
            dp[0][y] = dp[1][y] = (dp[0][y] || dp[1][y]);
          }
        }
        debug {
          if (L <= 10) {
            writeln(dp);
          }
        }
      }
      int ans = L + 1;
      foreach (y; 0 .. L + 1) if (dp[1][y]) {
        chmin(ans, abs((L - y) - y));
      }
      writeln(ans);
    }
  } catch (EOFException e) {
  }
}