#include <bits/stdc++.h>
using namespace std;
using lint = long long int;
using P = pair<int, int>;
using PL = pair<lint, lint>;
#define FOR(i, begin, end) for(int i=(begin),i##_end_=(end);i<i##_end_;i++)
#define IFOR(i, begin, end) for(int i=(end)-1,i##_begin_=(begin);i>=i##_begin_;i--)
#define REP(i, n) FOR(i,0,n)
#define IREP(i, n) IFOR(i,0,n)
#define ALL(a)  (a).begin(),(a).end()
constexpr int MOD = 1000000007;
constexpr lint B1 = 1532834020;
constexpr lint M1 = 2147482409;
constexpr lint B2 = 1388622299;
constexpr lint M2 = 2147478017;
constexpr int INF = 2147483647;
void yes(bool expr) {cout << (expr ? "Yes" : "No") << "\n";}
template<class T>void chmax(T &a, const T &b) { if (a<b) a=b; }
template<class T>void chmin(T &a, const T &b) { if (b<a) a=b; }
int main()
{
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  int N, M;
  cin >> N >> M;
  vector<lint> c(2*N*N, 1);
  vector<lint> d(2*N*N, 1e18);
  d[0] = 0;
  REP(i, M) {
    int h, w, cc;
    cin >> h >> w >> cc;
    h--; w--;
    c[h*N+w] += cc;
    c[h*N+w+N*N] += cc;
  }
  set<PL> st;
  st.insert(PL(0, 0));
  while(!st.empty()) {
    PL pl = *st.begin();
    st.erase(pl);
    lint dnow = pl.first;
    lint h = (pl.second % (N*N)) / N;
    lint w = pl.second % N;
    //cout << pl.second << " " << h << " " << w << endl;
    if(h != 0) {
      if(dnow + c[pl.second-N] < d[pl.second-N]) {
        d[pl.second-N] = dnow + c[pl.second-N];
        st.insert(PL(d[pl.second-N], pl.second-N));
      }
      if(pl.second < N*N && dnow + 1 < d[pl.second-N+N*N]) {
        d[pl.second-N+N*N] = dnow + 1;
        st.insert(PL(d[pl.second-N+N*N], pl.second-N+N*N));
      }
    }
    if(h != N-1) {
      if(dnow + c[pl.second+N] < d[pl.second+N]) {
        d[pl.second+N] = dnow + c[pl.second+N];
        st.insert(PL(d[pl.second+N], pl.second+N));
      }
      if(pl.second < N*N && dnow + 1 < d[pl.second+N+N*N]) {
        d[pl.second+N+N*N] = dnow + 1;
        st.insert(PL(d[pl.second+N+N*N], pl.second+N+N*N));
      }
    }
    if(w != 0) {
      if(dnow + c[pl.second-1] < d[pl.second-1]) {
        d[pl.second-1] = dnow + c[pl.second-1];
        st.insert(PL(d[pl.second-1], pl.second-1));
      }
      if(pl.second < N*N && dnow + 1 < d[pl.second-1+N*N]) {
        d[pl.second-1+N*N] = dnow + 1;
        st.insert(PL(d[pl.second-1+N*N], pl.second-1+N*N));
      }
    }
    if(w != N-1) {
      if(dnow + c[pl.second+1] < d[pl.second+1]) {
        d[pl.second+1] = dnow + c[pl.second+1];
        st.insert(PL(d[pl.second+1], pl.second+1));
      }
      if(pl.second < N*N && dnow + 1 < d[pl.second+1+N*N]) {
        d[pl.second+1+N*N] = dnow + 1;
        st.insert(PL(d[pl.second+1+N*N], pl.second+1+N*N));
      }
    }
  }
  cout << d[2*N*N-1] << endl;
}