import java.util.*; import java.io.*; import java.math.*;
public class Main{
	static void solve(){//Here is the main function
		ArrayList<Integer> 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<Integer> 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<Queue> 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;
			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){
							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){
							list[nextY][nextX].sum = nextSum;
							pq.add(new Queue(nextY, nextX, nextSum, false));
						}
						if(list[nextY][nextX].sum > tmpNextSum){
							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<Queue>{
	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<String> 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<Integer, String> 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<String> nextStrArray(){return myconv(next(), 8);}
	static ArrayList<String> nextCharArray(){return myconv(next(), 0);}
	static ArrayList<Integer> nextIntArray(){
		ArrayList<String> input = nextStrArray(); ArrayList<Integer> ret = new ArrayList<>(input.size());
		for(int i = 0; i < input.size(); i++){
			ret.add(Integer.parseInt(input.get(i)));
		}
		return ret;
	}
	static ArrayList<Long> nextLongArray(){
		ArrayList<String> input = nextStrArray(); ArrayList<Long> 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<String> myconv(String str, int no){//only split
		String splitString = CONVSTR.get(no);
		return new ArrayList<String>(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------------------------------------------
}