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; }
real readReal() { return readToken.to!real; }

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)); }


enum DX = [+1, 0, -1, 0];
enum DY = [0, +1, 0, -1];
enum INF = 10L^^18;

void main() {
  try {
    for (; ; ) {
      const N = readInt();
      const M = readInt();
      auto H = new int[M];
      auto W = new int[M];
      auto C = new long[M];
      foreach (i; 0 .. M) {
        H[i] = readInt() - 1;
        W[i] = readInt() - 1;
        C[i] = readLong();
      }
      
      auto cost = new long[][](N, N);
      foreach (i; 0 .. M) {
        cost[H[i]][W[i]] += C[i];
      }
      
      alias Entry = Tuple!(long, "c", int, "x", int, "y", int, "s");
      BinaryHeap!(Array!Entry, "a.c > b.c") que;
      auto dist = new long[][][](N, N, 2);
      foreach (x; 0 .. N) foreach (y; 0 .. N) {
        dist[x][y][] = INF;
      }
      chmin(dist[0][0][0], 0);
      que.insert(Entry(0, 0, 0, 0));
      for (; !que.empty; ) {
        const c = que.front.c;
        const x = que.front.x;
        const y = que.front.y;
        const s = que.front.s;
        que.removeFront;
        if (dist[x][y][s] == c) {
          foreach (dir; 0 .. 4) {
            const xx = x + DX[dir];
            const yy = y + DY[dir];
            if (0 <= xx && xx < N && 0 <= yy && yy < N) {
              {
                const cc = c + 1 + cost[xx][yy];
                if (chmin(dist[xx][yy][s], cc)) {
                  que.insert(Entry(cc, xx, yy, s));
                }
              }
              if (s == 0) {
                const cc = c + 1;
                if (chmin(dist[xx][yy][1], cc)) {
                  que.insert(Entry(cc, xx, yy, 1));
                }
              }
            }
          }
        }
      }
      
      long ans = INF;
      foreach (s; 0 .. 2) {
        chmin(ans, dist[N - 1][N - 1][s]);
      }
      writeln(ans);
    }
  } catch (EOFException e) {
  }
}