#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long double ld;
#define REP(i, n) for (int i = 0; i < (n); ++i)
#define REPR(i, n) for (int i = n - 1; i >= 0; --i)
#define FOR(i, m, n) for (int i = m; i < n; ++i)
#define FORR(i, m, n) for (int i = m; i >= n; --i)
#define ALL(v) (v).begin(),(v).end()
template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return 1; } return 0; }
const ll INF=1LL<<60;
const int inf=(1<<30)-1;
const int mod=1e9+7;
int dx[8]={1,0,-1,0,-1,-1,1,1};
int dy[8]={0,1,0,-1,-1,1,-1,1};
int main(){
    cin.tie(0);
    ios::sync_with_stdio(false);
    int n,m;cin >> n >> m;
    vector<vector<ll>> c(n,vector<ll>(n));
    REP(i,m){
        int h,w,C;cin >> h >> w >> C;
        h--,w--;
        c[h][w]=C;
    }
    vector<vector<vector<ll>>> d(n,vector<vector<ll>>(n,vector<ll>(2,INF)));
    d[0][0][0]=0;
    queue<pair<pair<int,int>,int>> q;
    q.push({{0,0},0});
    while(!q.empty()){
        auto p=q.front();q.pop();
        int u=p.first.first,v=p.first.second,w=p.second;
        REP(i,4){
            int x=u+dx[i],y=v+dy[i];
            if(x<0||x>=n||y<0||y>=n) continue;
            if(c[x][y]){
                if(chmin(d[x][y][w],d[u][v][w]+c[x][y]+1)){
                    q.push({{x,y},w});
                }
                if(!w&&chmin(d[x][y][1],d[u][v][0]+1)){
                    q.push({{x,y},1});
                }
            }
            else{
                if(chmin(d[x][y][w],d[u][v][w]+1)){
                    q.push({{x,y},w});
                }
            }
        }
    }
    cout << min(d[n-1][n-1][0],d[n-1][n-1][1]) << endl;
}