#include <bits/stdc++.h>

using namespace std;

using int64 = long long;
const int mod = 1e9 + 7;
// const int mod = 998244353;

const int64 infll = (1LL << 62) - 1;
const int inf = (1 << 30) - 1;

struct IoSetup {
  IoSetup() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(10);
    cerr << fixed << setprecision(10);
  }
} iosetup;


template< typename T1, typename T2 >
ostream &operator<<(ostream &os, const pair< T1, T2 > &p) {
  os << p.first << " " << p.second;
  return os;
}

template< typename T1, typename T2 >
istream &operator>>(istream &is, pair< T1, T2 > &p) {
  is >> p.first >> p.second;
  return is;
}

template< typename T >
ostream &operator<<(ostream &os, const vector< T > &v) {
  for(int i = 0; i < (int) v.size(); i++) {
    os << v[i] << (i + 1 != v.size() ? " " : "");
  }
  return os;
}

template< typename T >
istream &operator>>(istream &is, vector< T > &v) {
  for(T &in : v) is >> in;
  return is;
}

template< typename T1, typename T2 >
inline bool chmax(T1 &a, T2 b) { return a < b && (a = b, true); }

template< typename T1, typename T2 >
inline bool chmin(T1 &a, T2 b) { return a > b && (a = b, true); }

template< typename T = int64 >
vector< T > make_v(size_t a) {
  return vector< T >(a);
}

template< typename T, typename... Ts >
auto make_v(size_t a, Ts... ts) {
  return vector< decltype(make_v< T >(ts...)) >(a, make_v< T >(ts...));
}

template< typename T, typename V >
typename enable_if< is_class< T >::value == 0 >::type fill_v(T &t, const V &v) {
  t = v;
}

template< typename T, typename V >
typename enable_if< is_class< T >::value != 0 >::type fill_v(T &t, const V &v) {
  for(auto &e : t) fill_v(e, v);
}

template< typename F >
struct FixPoint : F {
  FixPoint(F &&f) : F(forward< F >(f)) {}

  template< typename... Args >
  decltype(auto) operator()(Args &&... args) const {
    return F::operator()(*this, forward< Args >(args)...);
  }
};

template< typename F >
inline decltype(auto) MFP(F &&f) {
  return FixPoint< F >{forward< F >(f)};
}


int main() {
  int N, M;
  cin >> N >> M;
  auto g = make_v< int >(N, N);
  for(int i = 0; i < M; i++) {
    int h, w, c;
    cin >> h >> w >> c;
    --h, --w;
    g[h][w] = c;
  }
  using pi = tuple< int64, int, int, bool >;
  priority_queue< pi, vector< pi >, greater<> > que;
  que.emplace(0, 0, 0, 0);
  auto dp = make_v< int64 >(N, N, 2);
  fill_v(dp, infll);
  dp[0][0][0] = 0;
  const int vy[] = {-1, 0, 1, 0};
  const int vx[] = {0, 1, 0, -1};

  while(!que.empty()) {
    int64 cost;
    int y, x;
    bool t;
    tie(cost, y, x, t) = que.top();
    que.pop();
    if(dp[y][x][t] < cost) continue;
    for(int i = 0; i < 4; i++) {
      int ny = y + vy[i];
      int nx = x + vx[i];
      if(ny < 0 || nx < 0 || ny >= N || nx >= N) continue;
      if(chmin(dp[ny][nx][t], cost + g[ny][nx] + 1)) {
        que.emplace(dp[ny][nx][t], ny, nx, t);
      }
    }
    if(!t) {
      for(int i = 0; i < 4; i++) {
        int ny = y + vy[i];
        int nx = x + vx[i];
        if(ny < 0 || nx < 0 || ny >= N || nx >= N) continue;
        if(chmin(dp[ny][nx][t ^ 1], cost + 1)) {
          que.emplace(dp[ny][nx][t ^ 1], ny, nx, t ^ 1);
        }
      }
    }
  }
  cout << dp[N - 1][N - 1][1] << "\n";
}