#include <bits/stdc++.h>
//#include <atcoder/all>
//using namespace atcoder;
const double pi = 3.141592653589793238462643383279;
using namespace std;
//typedef
//------------------------------------------
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef pair<long long, long long> PLL;
typedef pair<int, PII> TIII;
typedef long long LL;
typedef unsigned long long ULL;
typedef vector<LL> VLL;
typedef vector<VLL> VVLL;

//container util
//------------------------------------------
#define ALL(a) (a).begin(), (a).endf()
#define RALL(a) (a).rbegin(), (a).rend()
#define PB push_back
#define MP make_pair
#define SZ(a) int((a).size())
#define SQ(a) ((a) * (a))
#define EACH(i, c) for (typeof((c).begin()) i = (c).begin(); i != (c).endf(); ++i)
#define EXIST(s, e) ((s).find(e) != (s).endf())
#define SORT(c) sort((c).begin(), (c).endf())

//repetition
//------------------------------------------
#define FOR(i, s, n) for (int i = s; i < (int)n; ++i)
#define REP(i, n) FOR(i, 0, n)
#define MOD 1000000007

#define rep(i, a, b) for (int i = a; i < (b); ++i)
#define trav(a, x) for (auto &a : x)
#define all(x) x.begin(), x.end()
#define sz(x) (int)(x).size()

typedef long long ll;
typedef pair<int, int> pii;
typedef vector<int> vi;

#define chmin(x, y) x = min(x, y)
#define chmax(x, y) x = max(x, y)
const double EPS = 1e-4, PI = acos(-1);
//ここから編集    
typedef string::const_iterator State;

ll dp[510][510][2];
ll cost[510][510];
int dxy[5]={-1,0,1,0,-1};
int main()
{
    cin.tie(0);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(6);

    int N, M; cin >> N >> M;
    vector<ll> h(M), w(M), c(M);
    REP(i,510) REP(j,510) REP(k,2) dp[i][j][k] = (1LL<<60);
    dp[0][0][0] = 0;
    REP(i,M){
        cin >> h[i] >> w[i] >> c[i];
        h[i]--;
        w[i]--;
        cost[h[i]][w[i]] = c[i];
    }

    priority_queue<pair<ll, pair<ll, int>>,vector<pair<ll, pair<ll, int>>>, greater<pair<ll, pair<ll, int>>>> pq;
    pq.push(make_pair(0, make_pair(0, 0)));
    while(pq.size()){
        auto p = pq.top();
        pq.pop();

        int y = p.second.first/N, x = p.second.first%N;
        ll f = p.second.second;
        if(dp[y][x][f] < p.first) continue;
        for(int i=0; i<4; i++){
            int ny = y + dxy[i], nx = x + dxy[i+1];
            if(ny >= 0 && ny < N && nx >= 0 && nx < N){
                if(f==0){
                    if(dp[ny][nx][1]>dp[y][x][0]+1){
                        dp[ny][nx][1] = dp[y][x][0]+1;
                        pq.push(make_pair(dp[ny][nx][1], make_pair(ny*N+nx, 1)));
                    }
                    if(dp[ny][nx][0] > dp[y][x][0] + cost[ny][nx]+1){
                        dp[ny][nx][0] = dp[y][x][0] + cost[ny][nx]+1;
                        pq.push(make_pair(dp[ny][nx][0], make_pair(ny*N+nx, 0)));
                    }
                }else{
                    if(dp[ny][nx][1] > dp[y][x][1] + cost[ny][nx]+1){
                        dp[ny][nx][1] = dp[y][x][1] + cost[ny][nx]+1;
                        pq.push(make_pair(dp[ny][nx][1], make_pair(ny*N+nx, 1)));
                    }
                }
            }
        }
    }
    cout << min(dp[N-1][N-1][1], dp[N-1][N-1][0]) << endl;
    return 0;
}