#include <bits/stdc++.h>
using namespace std;
typedef long long int ll;
typedef pair<ll,ll> P;
typedef vector<ll> VI;
typedef vector<VI> VVI;
#define REP(i,n) for(int i=0;i<n;i++)
#define ALL(v) v.begin(),v.end()
const ll MOD=1000000007;
const ll INF=1e18;

struct xy {
	int x; int y; ll d; bool f;
	bool operator<(const xy &another) const {return d > another.d;};
};
int dx[4] = { 1,0,-1,0 }, dy[4] = { 0,1,0,-1 };
int n;
VVI c(510,VI(510,0));

ll bfs(int sx, int sy, int gx, int gy){
	priority_queue<xy> q;
	q.push({sx,sy,0,0});
	VVI dis0(n, VI(n, INF));
	VVI dis1(n, VI(n, INF));
	dis0[0][0]=0;
	while (!q.empty()) {
		int x=q.top().x, y=q.top().y; ll d=q.top().d;
		bool f=q.top().f;
		q.pop();
		if(!f&&d>dis0[x][y])
			continue;
		if(f&&d>dis1[x][y])
			continue;
		REP(k,4){
			int tox=x+dx[k], toy=y+dy[k];
			if(tox<0||tox>=n||toy<0||toy>=n)
				continue;
			if(!f){
				if(dis0[tox][toy]>d+1+c[tox][toy]){
					dis0[tox][toy]=d+1+c[tox][toy];
					q.push({tox,toy,d+1+c[tox][toy],0});
				}
				if(dis1[tox][toy]>d+1){
					dis1[tox][toy]=d+1;
					q.push({tox,toy,d+1,1});
				}
			}
			else{
				if(dis1[tox][toy]>d+1+c[tox][toy]){
					dis1[tox][toy]=d+1+c[tox][toy];
					q.push({tox,toy,d+1+c[tox][toy],1});
				}
			}
		}
	}
	return dis1[gx][gy];
}

int main(){
	int m; cin >> n >> m;
	int h, w, x;
	REP(i,m){
		cin >> h >> w >> x;
		h--, w--;
		c[h][w]=x;
	}
	cout << bfs(0,0,n-1,n-1) << endl;
	return 0;
}