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 Rust (1.15.1) (judge verdict: Accepted)

#![allow(unused_imports)]
#![allow(non_snake_case, unused)]

use std::cmp::*;
use std::collections::*;
use std::ops::*;

macro_rules! eprint {
	($($t:tt)*) => {{
		use ::std::io::Write;
		let _ = write!(::std::io::stderr(), $($t)*);
	}};
}
macro_rules! eprintln {
	() => { eprintln!(""); };
	($($t:tt)*) => {{
		use ::std::io::Write;
		let _ = writeln!(::std::io::stderr(), $($t)*);
	}};
}
macro_rules! dbg {
	($v:expr) => {{
		let val = $v;
		eprintln!("[{}:{}] {} = {:?}", file!(), line!(), stringify!($v), val);
		val
	}}
}

macro_rules! mat {
	($($e:expr),*) => { Vec::from(vec![$($e),*]) };
	($($e:expr,)*) => { Vec::from(vec![$($e),*]) };
	($e:expr; $d:expr) => { Vec::from(vec![$e; $d]) };
	($e:expr; $d:expr $(; $ds:expr)+) => { Vec::from(vec![mat![$e $(; $ds)*]; $d]) };
}

macro_rules! ok {
	($a:ident$([$i:expr])*.$f:ident()$(@$t:ident)*) => {
		$a$([$i])*.$f($($t),*)
	};
	($a:ident$([$i:expr])*.$f:ident($e:expr$(,$es:expr)*)$(@$t:ident)*) => { {
		let t = $e;
		ok!($a$([$i])*.$f($($es),*)$(@$t)*@t)
	} };
}

pub fn readln() -> String {
	let mut line = String::new();
	::std::io::stdin().read_line(&mut line).unwrap_or_else(|e| panic!("{}", e));
	line
}

macro_rules! read {
	($($t:tt),*; $n:expr) => {{
		let stdin = ::std::io::stdin();
		let ret = ::std::io::BufRead::lines(stdin.lock()).take($n).map(|line| {
			let line = line.unwrap();
			let mut it = line.split_whitespace();
			_read!(it; $($t),*)
		}).collect::<Vec<_>>();
		ret
	}};
	($($t:tt),*) => {{
		let line = readln();
		let mut it = line.split_whitespace();
		_read!(it; $($t),*)
	}};
}

macro_rules! _read {
	($it:ident; [char]) => {
		_read!($it; String).chars().collect::<Vec<_>>()
	};
	($it:ident; [u8]) => {
		Vec::from(_read!($it; String).into_bytes())
	};
	($it:ident; usize1) => {
		$it.next().unwrap_or_else(|| panic!("input mismatch")).parse::<usize>().unwrap_or_else(|e| panic!("{}", e)) - 1
	};
	($it:ident; [usize1]) => {
		$it.map(|s| s.parse::<usize>().unwrap_or_else(|e| panic!("{}", e)) - 1).collect::<Vec<_>>()
	};
	($it:ident; [$t:ty]) => {
		$it.map(|s| s.parse::<$t>().unwrap_or_else(|e| panic!("{}", e))).collect::<Vec<_>>()
	};
	($it:ident; $t:ty) => {
		$it.next().unwrap_or_else(|| panic!("input mismatch")).parse::<$t>().unwrap_or_else(|e| panic!("{}", e))
	};
	($it:ident; $($t:tt),+) => {
		($(_read!($it; $t)),*)
	};
}

pub fn main() {
	let _ = ::std::thread::Builder::new().name("run".to_string()).stack_size(32 * 1024 * 1024).spawn(run).unwrap().join();
}

const MOD: u32 = 1_000_000_007;
const INF: i64 = std::i64::MAX;

pub trait TE {
    type T: Clone;
    type E: Clone;
    fn fold(Self::T, Self::T) -> Self::T;
    fn eval(Self::T, Self::E) -> Self::T;
    fn merge(Self::E, Self::E) -> Self::E;
    fn e() -> Self::T;
    fn id() -> Self::E;
}

pub struct LazySegmentTree<R: TE> {
    size: usize,
    bit: usize,
    a: Vec<(R::T, R::E)>,
}

impl <R: TE> LazySegmentTree<R> {
    pub fn new(n: usize) -> LazySegmentTree<R> {
        let mut bit = 0;
        while (1 << bit) < n {
            bit += 1;
        }
        LazySegmentTree {
            size: 1 << bit,
            bit: bit,
            a: vec![(R::e(), R::id()); 2 << bit],
        }
    }
    pub fn build_by(z: &[R::T]) -> LazySegmentTree<R> {
        let n = z.len();
        let mut bit = 0;
        while (1 << bit) < n {
            bit += 1;
        }
        let mut a = vec![(R::e(), R::id()); 2 << bit];
        for (a, z) in a[(1 << bit)..].iter_mut().zip(z.iter()) {
            a.0 = z.clone();
        }
        for i in (1..(1 << bit)).rev() {
            let l = R::eval(a[2 * i].0.clone(), a[2 * i].1.clone());
            let r = R::eval(a[2 * i + 1].0.clone(), a[2 * i + 1].1.clone());
            a[i].0 = R::fold(l, r);
        }
        LazySegmentTree {
            size: 1 << bit,
            bit : bit,
            a: a,
        }
    }
    fn eval(&self, k: usize) -> R::T {
        R::eval(self.a[k].0.clone(), self.a[k].1.clone())
    }
    fn propagate(&mut self, x: usize) {
        let x = x + self.size;
        for i in (1..(self.bit + 1)).rev() {
            let k = x >> i;
            self.a[2 * k].1 = R::merge(self.a[2 * k].1.clone(), self.a[k].1.clone());
            self.a[2 * k + 1].1 = R::merge(self.a[2 * k + 1].1.clone(), self.a[k].1.clone());
            self.a[k].1 = R::id();
            self.a[k].0 = R::fold(self.eval(2 * k), self.eval(2 * k + 1));
        }
    }
    fn save(&mut self, x: usize) {
        let x = x + self.size;
        for i in 1..(self.bit + 1) {
            let k = x >> i;
            self.a[k].0 = R::fold(self.eval(2 * k), self.eval(2 * k + 1));
        }
    }
    pub fn update(&mut self, l: usize, r: usize, op: R::E) {
        self.propagate(l);
        self.propagate(r - 1);
        let mut x = l + self.size;
        let mut y = r + self.size;
        while x < y {
            if x & 1 == 1 {
                self.a[x].1 = R::merge(self.a[x].1.clone(), op.clone());
                x += 1;
            }
            if y & 1 == 1 {
                y -= 1;
                self.a[y].1 = R::merge(self.a[y].1.clone(), op.clone());
            }
            x >>= 1;
            y >>= 1;
        }
        self.save(l);
        self.save(r - 1);
    }
    pub fn find(&mut self, l: usize, r: usize) -> R::T {
        self.propagate(l);
        self.propagate(r - 1);
        let mut x = l + self.size;
        let mut y = r + self.size;
        let mut p = R::e();
        let mut q = R::e();
        while x < y {
            if x & 1 == 1 {
                p = R::fold(p, self.eval(x));
                x += 1;
            }
            if y & 1 == 1 {
                y -= 1;
                q = R::fold(self.eval(y), q);
            }
            x >>= 1;
            y >>= 1;
        }
        R::fold(p, q)
    }
}

struct R;
impl TE for R {
    type T = i64;
    type E = i64;
    fn fold(l: Self::T, r: Self::T) -> Self::T {
		// println!("l {} r {}", l, r);
        min(l,r)
    }
    fn eval(p: Self::T, x: Self::E) -> Self::T {
		// println!("p {} x {}", p, x);
        min(p,x)
    }
    fn merge(f: Self::E, g: Self::E) -> Self::E {
        min(f,g)
    }
    fn e() -> Self::T {
        INF
    }
    fn id() -> Self::E {
        INF
    }
}

fn solve() {
	let (n,m) = read!(usize,usize);
	let lrc = read!([usize];m);
	let mut range = vec![];
	for i in 0..m {
		let t = (lrc[i][0], lrc[i][1], lrc[i][2]);
		range.push(t);
	}
	range.sort();
	let mut s = LazySegmentTree::<R>::new(n+1);
	s.update(1,2,0);
	let mut rmax = 1;
	for i in 0..m {
		let (mut l,r,c) = range[i];
		if l>rmax {
			println!("{}", -1);
			return;
		}
		rmax = max(rmax, r);
		let cost = s.find(l,l+1) + c as i64;
		s.update(l, r+1, cost as i64);
	}
	if rmax!=n {
		println!("{}", -1);
		return;
	}
	println!("{}", s.find(n,n+1));

}

fn run() {
    solve();
}