#include <bits/stdc++.h>
#define fi first
#define se second
#define rep(i,n) for(int i = 0; i < (n); ++i)
#define rrep(i,n) for(int i = 1; i <= (n); ++i)
#define drep(i,n) for(int i = (n)-1; i >= 0; --i)
#define srep(i,s,t) for (int i = s; i < t; ++i)
#define rng(a) a.begin(),a.end()
#define rrng(a) a.rbegin(),a.rend()
#define maxs(x,y) (x = max(x,y))
#define mins(x,y) (x = min(x,y))
#define limit(x,l,r) max(l,min(x,r))
#define lims(x,l,r) (x = max(l,min(x,r)))
#define isin(x,l,r) ((l) <= (x) && (x) < (r))
#define pb push_back
#define eb emplace_back
#define sz(x) (int)(x).size()
#define pcnt __builtin_popcountll
#define uni(x) x.erase(unique(rng(x)),x.end())
#define snuke srand((unsigned)clock()+(unsigned)time(NULL));
#define show(x) cout<<#x<<" = "<<x<<endl;
#define PQ(T) priority_queue<T,v(T),greater<T> >
#define bn(x) ((1<<x)-1)
#define dup(x,y) (((x)+(y)-1)/(y))
#define newline puts("")
#define v(T) vector<T>
#define vv(T) v(v(T))
using namespace std;
typedef long long int ll;
typedef unsigned uint;
typedef unsigned long long ull;
typedef pair<int,int> P;
typedef tuple<int,int,int> T;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<ll> vl;
typedef vector<P> vp;
typedef vector<T> vt;
inline int in() { int x; scanf("%d",&x); return x;}
template<typename T>inline istream& operator>>(istream&i,v(T)&v)
{rep(j,sz(v))i>>v[j];return i;}
template<typename T>string join(const v(T)&v)
{stringstream s;rep(i,sz(v))s<<' '<<v[i];return s.str().substr(1);}
template<typename T>inline ostream& operator<<(ostream&o,const v(T)&v)
{if(sz(v))o<<join(v);return o;}
template<typename T1,typename T2>inline istream& operator>>(istream&i,pair<T1,T2>&v)
{return i>>v.fi>>v.se;}
template<typename T1,typename T2>inline ostream& operator<<(ostream&o,const pair<T1,T2>&v)
{return o<<v.fi<<","<<v.se;}
template<typename T>inline ll suma(const v(T)& a) { ll res(0); for (auto&& x : a) res += x; return res;}
const double eps = 1e-10;
const ll LINF = 1001002003004005006ll;
const int INF = 1001001001;
#define dame { puts("-1"); return 0;}
#define yn {puts("Yes");}else{puts("No");}
const int MX = 200005;
typedef v(vp) vvp;

struct F {
    typedef ll TT;
    TT d;
    F(TT d=LINF):d(d) {}
    F& operator+=(const F& f) {
        mins(d, f.d);
        return *this;
    }
    F operator+(const F& f) const{ return F(*this) += f;}
};

struct seg {
    vector<F> d; int n;
    seg(){}
    seg(int mx) {
        n=1; while (n<mx) n <<=1;
        d=vector<F>(n<<1);
    }
    void upd(int i){ d[i]=d[i<<1]+d[i<<1|1];}
    F& operator[](int i) {return d[n+1];}
    void init() { for(int i=n-1;i;--i) upd(i); }
    F get(int i) {
        F x;
        for (i+=n;i;i>>=1) x+=d[i];
        return x;
    }

    void add(int l, int r, F f){
        l+=n;r+=n;
        while (l<r) {
            if (l&1) d[l]+=f,++l;
            if (r&1) --r,d[r]+=f;
            l>>=1; r>>=1;
        }
    }
};

int main() {
    int n,m;
    scanf("%d%d",&n,&m);
    vvp e(n);
    rep(i,m) {
        int l,r,c;
        scanf("%d%d%d",&l,&r,&c);
        --l;
        e[l],eb(r,c);
    }
    seg t(n);
    t.add(0,1,F(0));
    ll ans =0;
    rep(i,n) {
        ll d=t.get(i).d;
        if (d==LINF) dame;
        ans=d;
        for (P p:e[i]) {
            t.add(i,p.fi,d+p.se);
        }
    }
    cout<<ans<<endl;
    return 0;
}
