#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#define pb push_back
#define mp make_pair
#define all(a) begin(a),end(a)
#define FOR(x,val,to) for(int x=(val);x<int((to));++x)
#define FORE(x,val,to) for(auto x=(val);x<=(to);++x)
#define FORR(x,arr) for(auto &x: arr)
#define FORS(x,plus,arr) for(auto x = begin(arr)+(plus); x != end(arr); ++x)
#define FORREV(x,plus,arr) for(auto x = (arr).rbegin()+(plus); x !=(arr).rend(); ++x)
#define REE(s_) {cout<<s_<<'\n';exit(0);}
#define GET(arr) for(auto &i: (arr)) sc(i)
#define whatis(x) cerr << #x << " is " << (x) << endl;
#define e1 first
#define e2 second
#define INF 0x7f7f7f7f
typedef std::pair<int,int> pi;
typedef std::vector<int> vi;
typedef std::vector<std::string> vs;
typedef int64_t ll;
typedef uint64_t ull;
#define umap unordered_map
#define uset unordered_set
using namespace std;
using namespace __gnu_pbds;
 
#ifdef ONLINE_JUDGE
#define whatis(x) ;
#endif
#ifdef _WIN32
#define getchar_unlocked() _getchar_nolock()
#define _CRT_DISABLE_PERFCRIT_LOCKS
#endif
template<class L, class R> ostream& operator<<(ostream &os, map<L, R> P) { for(auto const &vv: P)os<<"("<<vv.first<<","<<vv.second<<")"; return os; }
template<class T> ostream& operator<<(ostream &os, set<T> V) { os<<"[";for(auto const &vv:V)os<<vv<<","; os<<"]"; return os; }
template<class T> ostream& operator<<(ostream &os, vector<T> V) { os<<"[";for(auto const &vv:V)os<<vv<<","; os<<"]"; return os; }
template<class L, class R> ostream& operator<<(ostream &os, pair<L, R> P) { os<<"("<<P.first<<","<<P.second<<")"; return os; }
inline int fstoi(const string &str){auto it=str.begin();bool neg=0;int num=0;if(*it=='-')neg=1;else num=*it-'0';++it;while(it<str.end()) num=num*10+(*it++-'0');if(neg)num*=-1;return num;}
inline void getch(char &x){while(x = getchar_unlocked(), x < 33){;}}
inline void getstr(string &str){str.clear(); char cur;while(cur=getchar_unlocked(),cur<33){;}while(cur>32){str+=cur;cur=getchar_unlocked();}}
template<typename T> inline bool sc(T &num){ bool neg=0; int c; num=0; while(c=getchar_unlocked(),c<33){if(c == EOF) return false;} if(c=='-'){ neg=1; c=getchar_unlocked(); } for(;c>47;c=getchar_unlocked()) num=num*10+c-48; if(neg) num*=-1; return true;}template<typename T, typename ...Args> inline void sc(T &num, Args &...args){ bool neg=0; int c; num=0; while(c=getchar_unlocked(),c<33){;} if(c=='-'){ neg=1; c=getchar_unlocked(); } for(;c>47;c=getchar_unlocked()) num=num*10+c-48; if(neg) num*=-1; sc(args...); }
template<typename T> using ordered_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>; //s.find_by_order(), s.order_of_key() <- works like lower_bound
template<typename T> using ordered_map = tree<T, int, less<T>, rb_tree_tag, tree_order_statistics_node_update>;

const ll N = 1e5+2; //Sometimes must be scaled llo a power of 2!!!!!!!!!!!!!
ll t[N*2];
ll n;

ll iinf;
ll query(ll l, ll r){
    ll res = iinf;
    for(l+=n, r+=n; l<r; l>>=1, r>>=1){
        if(l&1)
            res = min(res,t[l++]);
            //res += t[l++];
        if(r&1)
            res = min(res,t[--r]);
            //res += t[--r];
    }
    return res;
}

void modify(ll p, ll val){
    for(t[p += n] = val; p > 1; p>>=1)
        t[p>>1] = min(t[p] , t[p^1]);
}
 
int main(){
    ios_base::sync_with_stdio(0);cin.tie(0);
    memset(&iinf,63,8);
    ll m;
    sc(n,m);
    memset(t,63,sizeof t);
    modify(n-1,0);
    array<ll,3> in[m];
    FORR(i,in){
        sc(i[0]);
        sc(i[1]);
        sc(i[2]);
        --i[0];
        --i[1];
    }
    sort(in,in+m,[](array<ll,3> f, array<ll,3> s){return f[1] > s[1];});
    FORR(i,in){
        /* FOR(i,0,n){ */
        /*     whatis(i) */
        /*     whatis(query(i,i+1)) */
        /* } */
        ll dx = query(i[0],i[1]+1)+i[2];
        if(dx < query(i[0],i[0]+1))
            modify(i[0],dx);
    }
    ll wh = query(0,1);
    cout << (wh == iinf ? -1 : wh) << '\n';
}



Measured memory_bytes: 4224