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 C++14 (GCC 5.4.1) (judge verdict: Accepted)

//https://atcoder.jp/contests/nikkei2019-2-qual/tasks/nikkei2019_2_qual_d

#include <iostream>
#include <vector>
#include <set>
#include <queue>
#include <map>
#include <cassert>
#include <algorithm>
#include <functional>
#include <iomanip>
#include <limits>
#include <stack>
#include <math.h>
#define REP(i,n) for(ll i=0, i##_len=(n); i<i##_len; ++i)
#define RREP(i,n) for(ll i=(n)-1; i>=0; --i)
#define SREP(i,s,n) for(ll i=(s), i##_len=(n); i<i##_len; ++i)
#define ALL(v) (v).begin(),(v).end()

using namespace std;
typedef long long int ll;
typedef unsigned long long int ull;

template <typename T>
T gcd(T a, T b) {
    if(a < b) gcd(b, a);
    if (b == 1) return 1;
    T r;
    while ((r=a%b)) {
        a = b;
        b = r;
    }
    return b;
}

bool comp(pair<ll, ll> a, pair<ll, ll> b) {
    return a.second < b.second;
}

struct UnionFind {
    vector<int> par;
    vector<int> rank;
    
    UnionFind(int n = 1) {
        init(n);
    }
    
    void init(int n = 1) {
        par.resize(n); rank.resize(n);
        for (int i = 0; i < n; ++i) {
            par[i] = i;
            rank[i] = 0;
        }
    }
    
    int root(int x) {
        if (par[x] == x) {
            return x;
        }
        else {
            int r = root(par[x]);
            return par[x] = r;
        }
    }
    
    bool issame(int x, int y) {
        return root(x) == root(y);
    }
    
    bool merge(int x, int y) {
        x = root(x); y = root(y);
        if (x == y) return false;
        if (rank[x] < rank[y]) swap(x, y);
        if (rank[x] == rank[y]) ++rank[x];
        par[y] = x;
        return true;
    }
};

#define MOD (998244353)
#define INF (ll(1e18))

class SegmentTree {
public:
    int N, n;
    vector<ll> value;
    SegmentTree(int n) {
        this->n = n;
        this->N = 1;
        while (N < n) N *= 2;
        this->value = vector<ll>(2 * N - 1, INF);
    }
    
    void update(int i, ll x) {
        i += N - 1;
        value[i] = x;
        while (i > 0) {
            i = (i - 1) / 2;
            value[i] = min(value[i * 2 + 1], value[i * 2 + 2]);
        }
    }
    
    ll query(int l, int r) {
        return _query(l, r, 0, 0, N);
    }
    
    ll _query(int a, int b, int k, int l, int r) {
        if (r <= a || b <= l) return INF;
        if (a <= l && r <= b)
            return value[k];
        else {
            ll c1 = _query(a, b, 2 * k + 1, l, (l + r) / 2);
            ll c2 = _query(a, b, 2 * k + 2, (l + r) / 2, r);
            return min(c1, c2);
        }
    }
};

int main(int argc, const char * argv[]) {
    cin.tie(0);
    ios::sync_with_stdio(false);

    ll N, M;
    cin >> N >> M;
    vector<pair<pair<ll, ll>, ll>> LRC(M);
    REP(i, M) {
        cin >> LRC[i].first.first >> LRC[i].first.second >> LRC[i].second;
    }
    sort(ALL(LRC));
    
    SegmentTree st(N);
    st.update(0, 0);
    REP(i, M) {
        ll L = LRC[i].first.first-1;
        ll R = LRC[i].first.second-1;
        ll C = LRC[i].second;
        
        ll Q0 = st.query(L, R+1);
        ll Q1 = st.query(R, R+1);
        
        ll x = min(Q0+C, Q1);
        st.update(R, x);
    }
    
    ll score = st.query(N-1, N);
    cout << (score>=INF?-1:score) << endl;
}