Problem:
Score : 600 points

Problem Statement

We have N points numbered 1 to N arranged in a line in this order.

Takahashi decides to make an undirected graph, using these points as the vertices.
In the beginning, the graph has no edge. Takahashi will do M operations to add edges in this graph.
The i-th operation is as follows:

The operation uses integers L_i and R_i between 1 and N (inclusive), and a positive integer C_i. For every pair of integers (s, t) such that L_i \leq s < t \leq R_i, add an edge of length C_i between Vertex s and Vertex t.

The integers L_1, ..., L_M, R_1, ..., R_M, C_1, ..., C_M are all given as input.

Takahashi wants to solve the shortest path problem in the final graph obtained. Find the length of the shortest path from Vertex 1 to Vertex N in the final graph.

Constraints

2 \leq N \leq 10^5

1 \leq M \leq 10^5

1 \leq L_i < R_i \leq N

1 \leq C_i \leq 10^9

Input

Input is given from Standard Input in the following format:

N M
L_1 R_1 C_1
:
L_M R_M C_M

Output

Print the length of the shortest path from Vertex 1 to Vertex N in the final graph.
If there is no shortest path, print -1 instead.

Sample Input 1

4 3
1 3 2
2 4 3
1 4 6

Sample Output 1

5

We have an edge of length 2 between Vertex 1 and Vertex 2, and an edge of length 3 between Vertex 2 and Vertex 4, so there is a path of length 5 between Vertex 1 and Vertex 4.

Sample Input 2

4 2
1 2 1
3 4 2

Sample Output 2

-1

Sample Input 3

10 7
1 5 18
3 4 8
1 3 5
4 7 10
5 9 8
6 10 5
8 10 3

Sample Output 3

28

Solution in Java8 (OpenJDK 1.8.0) (judge verdict: Time Limit Exceeded)

import java.io.*;
import java.util.*;

public class Main {
    static long mod = (int)Math.pow(10,9)+7;
    static long max = Long.MAX_VALUE/5;
	public static void main(String[] args) throws IOException {
		FastScanner sc = new FastScanner(System.in);
		PrintWriter pw = new PrintWriter(System.out);
		int n = sc.nextInt();
		int m = sc.nextInt();
		ArrayList<HashMap<Integer,Long>> a = new ArrayList<>();
		for(int i = 0; i < n; i++){
		    a.add(new HashMap<Integer,Long>());
		}
		for(int i = 1; i < n; i++){
		    a.get(i).put(i-1,0L);
		}
		for(int i = 0; i < m; i++){
		    int l = sc.nextInt()-1;
		    int r = sc.nextInt()-1;
		    long c = sc.nextLong();
		    a.get(l).put(r,c);
		}
		long ans = dijkstra(a,0,n-1,n);
		System.out.println(ans != max ? ans : -1);
	}
	
	public static long dijkstra(ArrayList<HashMap<Integer,Long>> cost , int s, int end,int node) {
        long[] d = new long[node];
        Arrays.fill(d,Long.MAX_VALUE);
        boolean[] used = new boolean[node];
        d[s] = 0;
        while(true){
            int v = -1;
            for(int i = 0; i < node; i++){
                if(!used[i] && v == -1){
                    v = i;
                }else if(!used[i] &&  d[i] < d[v]){
                    v = i;
                }
            }
            if(v == -1) break;
            used[v] = true;
            for(int j = 0; j < node; j++){
                long c = cost.get(v).getOrDefault(j,max);
                d[j] = Math.min(d[j],d[v]+c);
            }
        }
        return d[end];
    }
}

class FastScanner {
    private BufferedReader reader = null;
    private StringTokenizer tokenizer = null;
    public FastScanner(InputStream in) {
        reader = new BufferedReader(new InputStreamReader(in));
        tokenizer = null;
    }

    public String next() {
        if (tokenizer == null || !tokenizer.hasMoreTokens()) {
            try {
                tokenizer = new StringTokenizer(reader.readLine());
            } catch (IOException e) {
                throw new RuntimeException(e);
            }
        }
        return tokenizer.nextToken();
    }

    public String nextLine() {
        if (tokenizer == null || !tokenizer.hasMoreTokens()) {
            try {
                return reader.readLine();
            } catch (IOException e) {
                throw new RuntimeException(e);
            }
        }
        return tokenizer.nextToken("\n");
    }

    public long nextLong() {
        return Long.parseLong(next());
    }

    public int nextInt() {
        return Integer.parseInt(next());
    }

    public double nextDouble() {
         return Double.parseDouble(next());
    }

    public int[] nextIntArray(int n) {
        int[] a = new int[n];
        for (int i = 0; i < n; i++)
            a[i] = nextInt();
        return a;
    }

    public long[] nextLongArray(int n) {
        long[] a = new long[n];
        for (int i = 0; i < n; i++)
            a[i] = nextLong();
        return a;
    } 
}