#include<bits/stdc++.h>
using namespace std;
#define ALL(x) begin(x),end(x)
#define rep(i,n) for(int i=0;i<(n);i++)
#define debug(v) cout<<#v<<":";for(auto x:v){cout<<x<<' ';}cout<<endl;
#define mod 1000000007
using ll=long long;
const int INF=1000000000;
const ll LINF=1001002003004005006ll;
int dx[]={1,0,-1,0},dy[]={0,1,0,-1};
// ll gcd(ll a,ll b){return b?gcd(b,a%b):a;}
template<class T>bool chmax(T &a,const T &b){if(a<b){a=b;return true;}return false;}
template<class T>bool chmin(T &a,const T &b){if(b<a){a=b;return true;}return false;}

struct IOSetup{
    IOSetup(){
        cin.tie(0);
        ios::sync_with_stdio(0);
        cout<<fixed<<setprecision(12);
    }
} iosetup;
 
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==(int)v.size()?"":" ");
    return os;
}
template<typename T>
istream &operator>>(istream &is,vector<T>&v){
    for(T &x:v)is>>x;
    return is;
}


signed main(){
    int n,m;cin>>n>>m;
    vector<vector<ll>> cost(n,vector<ll>(n,0));
    rep(_,m){
        int i,j,c;cin>>i>>j>>c;i--,j--;
        cost[i][j]=c;
    }

    ll dp[n][n][2];
    rep(i,n)rep(j,n)rep(k,2)dp[i][j][k]=LINF;
    dp[0][0][0]=0;
    using Q=tuple<ll,int,int,int>;
    priority_queue<Q,vector<Q>,greater<Q>> que;
    que.emplace(0,0,0,0);
    while(!que.empty()){
        auto [s,i,j,k]=que.top();
        que.pop();
        rep(l,4){
            int ni=i+dx[l],nj=j+dy[l];
            if(not (0<=ni and ni<n and 0<=nj and nj<n)) continue;
            if(chmin(dp[ni][nj][k],s+1+cost[ni][nj])){
                que.emplace(dp[ni][nj][k],ni,nj,k);
            }
            if(k==0){
                if(chmin(dp[ni][nj][1],s+1)){
                    que.emplace(dp[ni][nj][1],ni,nj,1);
                }
            }
        }
    }
    cout<<dp[n-1][n-1][1]<<endl;
    return 0;
}