#include <bits/stdc++.h>
using namespace std;
#define LOCAL
#pragma region Macros
typedef long long ll;
#define ALL(x) (x).begin(),(x).end()
const long long MOD=1000000007;
// const long long MOD=998244353;
const int INF=1e9;
const long long IINF=1e18;
const int dx[4]={1,0,-1,0},dy[4]={0,1,0,-1};
const char dir[4]={'D','R','U','L'};

template<typename T>
istream &operator>>(istream &is,vector<T> &v){
    for (T &x:v) is >> x;
    return is;
}
template<typename T>
ostream &operator<<(ostream &os,const vector<T> &v){
    for (int i=0;i<v.size();++i){
        os << v[i] << (i+1==v.size()?"": " ");
    }
    return os;
}
template<typename T,typename U>
ostream &operator<<(ostream &os,const pair<T,U> &p){
    os << '(' << p.first << ',' << p.second << ')';
    return os;
}
template<typename T,typename U>
ostream &operator<<(ostream &os,const map<T,U> &m){
    os << '{';
    for (auto itr=m.begin();itr!=m.end();++itr){
        os << '(' << itr->first << ',' << itr->second << ')';
        if (++itr!=m.end()) os << ',';
        --itr;
    }
    os << '}';
    return os;
}
template<typename T>
ostream &operator<<(ostream &os,const set<T> &s){
    os << '{';
    for (auto itr=s.begin();itr!=s.end();++itr){
        os << *itr;
        if (++itr!=s.end()) os << ',';
        --itr;
    }
    os << '}';
    return os;
}

void debug_out(){cerr << '\n';}
template<class Head,class... Tail>
void debug_out(Head&& head,Tail&&... tail){
    cerr << head;
    if (sizeof...(Tail)>0) cerr << ", ";
    debug_out(move(tail)...);
}
#ifdef LOCAL
#define debug(...) cerr << " ";\
cerr << #__VA_ARGS__ << " :[" << __LINE__ << ":" << __FUNCTION__ << "]" << '\n';\
cerr << " ";\
debug_out(__VA_ARGS__)
#else
#define debug(...) 42
#endif

template<typename T> T gcd(T x,T y){return y!=0?gcd(y,x%y):x;}
template<typename T> T lcm(T x,T y){return x/gcd(x,y)*y;}

template<class T1,class T2> inline bool chmin(T1 &a,T2 b){
    if (a>b){a=b; return true;} return false;
}
template<class T1,class T2> inline bool chmax(T1 &a,T2 b){
    if (a<b){a=b; return true;} return false;
}
#pragma endregion

using T=tuple<ll,int,int,int>;
const int MAX_N=510;
ll dp[MAX_N][MAX_N][2];

int main(){
    cin.tie(0);
    ios::sync_with_stdio(false);
    int N,M; cin >> N >> M;
    vector<vector<ll>> cost(N,vector<ll>(N,1));
    for (int i=0;i<M;++i){
        int h,w,c; cin >> h >> w >> c;
        cost[--h][--w]+=c;
    }

    for (int i=0;i<N;++i){
        for (int j=0;j<N;++j){
            for (int k=0;k<2;++k){
                dp[i][j][k]=IINF;
            }
        }
    }
    priority_queue<T,vector<T>,greater<T>> pq;
    dp[0][0][0]=0; pq.emplace(dp[0][0][0],0,0,0);

    while (!pq.empty()){
        auto t=pq.top(); pq.pop();
        int x=get<1>(t),y=get<2>(t),z=get<3>(t);
        if (dp[x][y][z]<get<0>(t)) continue;
        for (int i=0;i<4;++i){
            int nx=x+dx[i],ny=y+dy[i];
            if (nx<0||N<=nx||ny<0||N<=ny) continue;
            if (chmin(dp[nx][ny][z],dp[x][y][z]+cost[nx][ny])){
                pq.emplace(dp[nx][ny][z],nx,ny,z);
            }
            if (!z&&chmin(dp[nx][ny][z+1],dp[x][y][z]+1)){
                pq.emplace(dp[nx][ny][z+1],nx,ny,z+1);
            }
        }
    }

    ll ans=min(dp[N-1][N-1][0],dp[N-1][N-1][1]);
    cout << ans << '\n';
}