import java.util.*; import java.io.*; import java.math.*; public class Main{ static void solve(){//Here is the main function ArrayList one = nextIntArray(); int N = one.get(0); int M = one.get(1); Node[][] list = new Node[N][N]; for(int i = 0; i < N; i++){ for(int j = 0; j < N; j++){ list[i][j] = new Node(); } } for(int i = 0; i < M; i++){ ArrayList tmp = nextIntArray(); int H = tmp.get(0) - 1; int W = tmp.get(1) - 1; int C = tmp.get(2); list[H][W].cost = C; } PriorityQueue pq = new PriorityQueue<>(); list[0][0].sum = 0; pq.add(new Queue(0,0,new Long("0"), false)); int[] dy = {-1,0,1,0}; int[] dx = {0,-1,0,1}; while(pq.size() > 0){ Queue q = pq.poll(); int nowY = q.y; int nowX = q.x; long nowSum = q.sum; if(list[nowY][nowX].sum <= nowSum){ continue; } list[nowY][nowX].sum = nowSum; boolean usedFree = q.usedFree; for(int i = 0; i < dy.length; i++){ int nextY = nowY + dy[i]; int nextX = nowX + dx[i]; try{ long nextSum = nowSum + 1 + list[nextY][nextX].cost; if(usedFree){ if(list[nextY][nextX].sum > nextSum){ pq.add(new Queue(nextY, nextX, nextSum, true)); } }else{ long tmpNextSum = nextSum - list[nextY][nextX].cost; if(list[nextY][nextX].sum > nextSum){ pq.add(new Queue(nextY, nextX, nextSum, false)); } if(list[nextY][nextX].sum > tmpNextSum){ pq.add(new Queue(nextY, nextX, tmpNextSum, true)); } } }catch(Exception e){ //out index } } } //myerr(list[1][2].sum); myout(list[N - 1][N - 1].sum); } //Method addition frame start static class Queue implements Comparable{ boolean usedFree; long sum; int y; int x; Queue(int y, int x, long sum, boolean usedFree){ this.y = y; this.x = x; this.sum = sum; this.usedFree = usedFree; } public int compareTo(Queue ato){ return Long.compare(this.sum, ato.sum); } } static class Node{ long sum = 1l << 60; int cost = 0; Node(){} } //Method addition frame end //Don't have to see. start------------------------------------------ static class InputIterator{ ArrayList inputLine = new ArrayList<>(1024); int index = 0; int max; String read; InputIterator(){ BufferedReader br = new BufferedReader(new InputStreamReader(System.in)); try{ while((read = br.readLine()) != null){ inputLine.add(read); } }catch(IOException e){} max = inputLine.size(); } boolean hasNext(){return (index < max);} String next(){ if(hasNext()){ return inputLine.get(index++); }else{ throw new IndexOutOfBoundsException("There is no more input"); } } } static HashMap CONVSTR = new HashMap<>(); static InputIterator ii = new InputIterator();//This class cannot be used in reactive problem. static PrintWriter out = new PrintWriter(System.out); static void flush(){out.flush();} static void myout(Object t){out.println(t);} static void myerr(Object t){System.err.print("debug:");System.err.println(t);} static String next(){return ii.next();} static boolean hasNext(){return ii.hasNext();} static int nextInt(){return Integer.parseInt(next());} static long nextLong(){return Long.parseLong(next());} static double nextDouble(){return Double.parseDouble(next());} static ArrayList nextStrArray(){return myconv(next(), 8);} static ArrayList nextCharArray(){return myconv(next(), 0);} static ArrayList nextIntArray(){ ArrayList input = nextStrArray(); ArrayList ret = new ArrayList<>(input.size()); for(int i = 0; i < input.size(); i++){ ret.add(Integer.parseInt(input.get(i))); } return ret; } static ArrayList nextLongArray(){ ArrayList input = nextStrArray(); ArrayList ret = new ArrayList<>(input.size()); for(int i = 0; i < input.size(); i++){ ret.add(Long.parseLong(input.get(i))); } return ret; } @SuppressWarnings("unchecked") static String myconv(Object list, int no){//only join String joinString = CONVSTR.get(no); if(list instanceof String[]){ return String.join(joinString, (String[])list); }else if(list instanceof ArrayList){ return String.join(joinString, (ArrayList)list); }else{ throw new ClassCastException("Don't join"); } } static ArrayList myconv(String str, int no){//only split String splitString = CONVSTR.get(no); return new ArrayList(Arrays.asList(str.split(splitString))); } public static void main(String[] args){ CONVSTR.put(8, " "); CONVSTR.put(9, "\n"); CONVSTR.put(0, ""); solve();flush(); } //Don't have to see. end------------------------------------------ }