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 Julia (0.5.0) (judge verdict: Accepted)

parseInt(x) = parse(Int, x)
parseMap(x::Array{SubString{String},1}) = map(parseInt, x)
function eval(a::Tuple{Int,Int},b::Tuple{Int,Int})
	if a[2]<b[2]
		1
	elseif a[2]==b[2]
		0
	else
		-1
	end
end

function hpush(h::Array{Tuple{Int,Int},1},x::Tuple{Int,Int})
	push!(h,x)
	if length(h) > 1
		l = length(h)
		k = l>>1
		while eval(x,h[k])==1&&k>=1
			tmp = h[k]
			h[k] = x
			h[l] = tmp
			k = max(k>>1,1)
			l = l>>1
		end
	end
end

function hpop(h::Array{Tuple{Int,Int},1})
	x = h[1]
	z = pop!(h)
	if !isempty(h)
		h[1] = z
		k = 1
		l = length(h)
		while k <= l
			if 2*k>l
				break
			elseif 2*k+1>l
				if eval(h[2*k],z)>=0
					break
				else
					tmp = h[2*k]
					h[2*k] = z
					h[k] = tmp
					k = 2*k
				end
			else
				if eval(z,h[2*k])>=0&&eval(z,h[2*k+1])>=0
					break
				else
					t = eval(h[2*k],h[2*k+1])>=0?2*k:2*k+1
					tmp = h[t]
					h[t] = z
					h[k] = tmp
					k = t
				end
			end
		end
	end
	x
end

function main()
    n,m = readline() |> split |> parseMap
    e = [Tuple{Int,Int}[] for i in 1:n]
    for i in 2:n
        push!(e[i],(i-1,0))
    end
    for i in 1:m
        l,r,c = readline() |> split |> parseMap
        push!(e[l],(r,c))
        push!(e[r],(l,c))
    end
    dis = -ones(Int, length(e))
	h = Array{Tuple{Int,Int},1}()
	push!(h,(1,0))
	while !isempty(h)
		v,w = hpop(h)
		dis[v] = dis[v]<0?w:min(dis[v],w)
		for i in 1:length(e[v])
			if dis[e[v][i][1]]<0　|| dis[e[v][i][1]]>w+e[v][i][2]
				dis[e[v][i][1]] = w+e[v][i][2]
				hpush(h,(e[v][i][1],w+e[v][i][2]))
			end
		end
	end
	println(dis[n])   
end

main()