#include <bits/stdc++.h>
using namespace std;
//#define int long long
//TEMPLATE START---------------8<---------------8<---------------8<---------------8<---------------//
typedef long long ll;       typedef long double ld;  typedef pair<int,int> pii; typedef pair<ll,ll> pll;  typedef vector<int> vi;   typedef vector<ll> vl;
typedef vector<string> vst; typedef vector<bool> vb; typedef vector<ld> vld;    typedef vector<pii> vpii; typedef vector<pll> vpll; typedef vector<vector<int> > vvi;
const int INF = (0x7FFFFFFFL); const ll INFF = (0x7FFFFFFFFFFFFFFFL); const string ALPHABET = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
const int MOD = 1e9 + 7;       const int MODD = 998244353;            const string alphabet = "abcdefghijklmnopqrstuvwxyz";
const double PI = acos(-1.0);  const double EPS = 1e-9;               const string Alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
int dx[9] = { 1, 0, -1,  0,  1, -1, -1, 1, 0 };
int dy[9] = { 0, 1,  0, -1, -1, -1,  1, 1, 0 };
#define ln '\n'
#define scnaf scanf
#define sacnf scanf
#define sancf scanf
#define SS(type, ...)type __VA_ARGS__;MACRO_VAR_Scan(__VA_ARGS__);
template<typename T> void MACRO_VAR_Scan(T& t){cin >> t;}template<typename First, typename...Rest> void MACRO_VAR_Scan(First& first, Rest&...rest){cin >> first;MACRO_VAR_Scan(rest...);}
#define SV(type,c,n) vector<type> c(n);for(auto& i:c)cin >> i;
#define SVV(type,c,n,m) vector<vector<type>> c(n,vector<type>(m));for(auto& r:c)for(auto& i:r)cin >> i;
template<class T,class U>ostream &operator<<(ostream &o,const pair<T,U>&j){o<<"{"<<j.first<<", "<<j.second<<"}";return o;}
template<class T,class U>ostream &operator<<(ostream &o,const map<T,U>&j){o<<"{";for(auto t=j.begin();t!=j.end();++t)o<<(t!=j.begin()?", ":"")<<*t;o<<"}";return o;}
template<class T>ostream &operator<<(ostream &o,const set<T>&j){o<<"{";for(auto t=j.begin();t!=j.end();++t)o<<(t!=j.begin()?", ":"")<<*t;o<<"}";return o;}
template<class T>ostream &operator<<(ostream &o,const multiset<T>&j){o<<"{";for(auto t=j.begin();t!=j.end();++t)o<<(t!=j.begin()?", ":"")<<*t;o<<"}";return o;}
template<class T>ostream &operator<<(ostream &o,const vector<T>&j){o<<"{";for(int i=0;i<(int)j.size();++i)o<<(i>0?", ":"")<<j[i];o<<"}";return o;}
inline int print(void){cout << endl; return 0;}
template<class Head> int print(Head&& head){cout << head;print();return 0;} template<class Head,class... Tail> int print(Head&& head,Tail&&... tail){cout<<head<<" ";print(forward<Tail>(tail)...);return 0;}
inline int debug(void){cerr << endl; return 0;}
template<class Head> int debug(Head&& head){cerr << head;debug();return 0;} template<class Head,class... Tail> int debug(Head&& head,Tail&&... tail){cerr<<head<<" ";debug(forward<Tail>(tail)...);return 0;}
template<typename T> void PA(T &a){int ASIZE=sizeof(a)/sizeof(a[0]);for(int ii=0;ii<ASIZE;++ii){cout<<a[ii]<<" \n"[ii==ASIZE-1];}}
template<typename T> void PV(T &v){int VSIZE=v.size();for(int ii=0;ii<VSIZE;++ii){cout<<v[ii]<<" \n"[ii==VSIZE-1];}}
#define ER(x)  cerr << #x << " = " << (x) << endl;
#define ERV(v) {cerr << #v << " : ";for(const auto& xxx : v){cerr << xxx << " ";}cerr << "\n";}
inline int YES(bool x){cout<<((x)?"YES":"NO")<<endl;return 0;} inline int Yes(bool x){cout<<((x)?"Yes":"No")<<endl;return 0;}  inline int yes(bool x){cout<<((x)?"yes":"no")<<endl;return 0;}
inline int yES(bool x){cout<<((x)?"yES":"nO")<<endl;return 0;} inline int Yay(bool x){cout<<((x)?"Yay!":":(")<<endl;return 0;}
template<typename A,typename B> void sankou(bool x,A a,B b){cout<<((x)?(a):(b))<<endl;}
#define _overload3(_1,_2,_3,name,...) name
#define _REP(i,n) REPI(i,0,n)
#define REPI(i,a,b) for(ll i=ll(a);i<ll(b);++i)
#define REP(...) _overload3(__VA_ARGS__,REPI,_REP,)(__VA_ARGS__)
#define _RREP(i,n) RREPI(i,n,0)
#define RREPI(i,a,b) for(ll i=ll(a);i>=ll(b);--i)
#define RREP(...) _overload3(__VA_ARGS__,RREPI,_RREP,)(__VA_ARGS__)
#define EACH(e,v) for(auto& e : v)
#define PERM(v) sort((v).begin(),(v).end());for(bool c##p=1;c##p;c##p=next_permutation((v).begin(),(v).end()))
#define ADD(a,b) a=(a+ll(b))%MOD
#define MUL(a,b) a=(a*ll(b))%MOD
inline ll MOP(ll x,ll n,ll m=MOD){ll r=1;while(n>0){if(n&1)(r*=x)%=m;(x*=x)%=m;n>>=1;}return r;}
inline ll gcd(ll a,ll b){return b?gcd(b,a%b):a;}inline ll lcm(ll a,ll b){return a*b/gcd(a,b);}inline ll POW(ll a,ll b){ll c=1ll;do{if(b&1)c*=1ll*a;a*=1ll*a;}while(b>>=1);return c;}
template<typename T,typename A,typename B> inline bool between(T x,A a,B b) {return ((a<=x)&&(x<b));}template<class T> inline T sqr(T x){return x*x;}
template<typename A,typename B> inline bool chmax(A &a,const B &b){if(a<b){a=b;return 1;}return 0;}
template<typename A,typename B> inline bool chmin(A &a,const B &b){if(a>b){a=b;return 1;}return 0;}
#define tmax(x,y,z) max((x),max((y),(z)))
#define tmin(x,y,z) min((x),min((y),(z)))
#define PB push_back
#define MP make_pair
#define MT make_tuple
#define all(v) (v).begin(),(v).end()
#define rall(v) (v).rbegin(),(v).rend()
#define SORT(v) sort((v).begin(),(v).end())
#define RSORT(v) sort((v).rbegin(),(v).rend())
#define EXIST(s,e) (find((s).begin(),(s).end(),(e))!=(s).end())
#define EXISTST(s,c) (((s).find(c))!=string::npos)
#define POSL(x,val) (lower_bound(x.begin(),x.end(),val)-x.begin())
#define POSU(x,val) (upper_bound(x.begin(),x.end(),val)-x.begin())
#define GEQ(x,val) (int)(x).size() - POSL((x),(val))
#define GREATER(x,val) (int)(x).size() - POSU((x),(val))
#define LEQ(x,val) POSU((x),(val))
#define LESS(x,val) POSL((x),(val))
#define SZV(a) int((a).size())
#define SZA(a) sizeof(a)/sizeof(a[0])
#define ZERO(a) memset(a,0,sizeof(a))
#define MINUS(a) memset(a,0xff,sizeof(a))
#define MEMINF(a) memset(a,0x3f,sizeof(a))
#define FILL(a,b) memset(a,b,sizeof(a))
#define UNIQUE(v) sort((v).begin(),(v).end());(v).erase(unique((v).begin(),(v).end()),(v).end())
struct abracadabra{
    abracadabra(){
        cin.tie(0); ios::sync_with_stdio(0);
        cout << fixed << setprecision(20);
        cerr << fixed << setprecision(5);
    };
} ABRACADABRA;

//TEMPLATE END---------------8<---------------8<---------------8<---------------8<---------------//

#define N 100003

// Segment Tree Beats
// - l<=i<r について、 A_i の値を min(A_i, x) に更新
// - l<=i<r について、 A_i の値を max(A_i, x) に更新
// - l<=i<r の中の A_i の最大値を求める
// - l<=i<r の中の A_i の最小値を求める
// - l<=i<r の A_i の和を求める
// - l<=i<r について、 A_i の値に x を加える
// - l<=i<r について、 A_i の値を x に更新

// 各ノードごとに構造体にまとめる形で実装 (このように実装しないとTLEした)
class SegmentTree {
  static const ll inf = 1e18;
  struct Node {
    Node *left, *right;
    ll max_v, smax_v, max_c;
    ll min_v, smin_v, min_c;
    ll sum;
    ll len, ladd, lval;

    Node() : left(0), right(0), ladd(0), lval(inf) {}

    void init(ll x) {
      max_v = min_v = sum = x;
      smax_v = -inf; smin_v = inf;
      max_c = min_c = 1;
    }

    void init_empty() {
      max_v = smax_v = -inf;
      min_v = smin_v = inf;
      max_c = min_c = 0;
    }

    void update_max(ll x) {
      sum += (x - max_v) * max_c;

      if(max_v == min_v) {
        max_v = min_v = x;
      } else if(max_v == smin_v) {
        max_v = smin_v = x;
      } else {
        max_v = x;
      }

      if(lval != inf && x < lval) {
        lval = x;
      }
    }

    void update_min(ll x) {
      sum += (x - min_v) * min_c;

      if(max_v == min_v) {
        max_v = min_v = x;
      } else if(max_v == smin_v) {
        min_v = smax_v = x;
      } else {
        min_v = x;
      }

      if(lval != inf && lval < x) {
        lval = x;
      }
    }

    void addall(ll x) {
      max_v += x;
      if(smax_v != -inf) smax_v += x;
      min_v += x;
      if(smin_v != inf) smin_v += x;

      sum += len * x;
      if(lval != inf) {
        lval += x;
      } else {
        ladd += x;
      }
    }

    void updateall(ll x) {
      max_v = min_v = x;
      smax_v = -inf; smin_v = inf;
      max_c = min_c = len;

      sum = len * x;
      lval = x; ladd = 0;
    }

    void push() {

      if(lval != inf) {
        left->updateall(lval);
        right->updateall(lval);
        lval = inf;
        return;
      }

      if(ladd != 0) {
        left->addall(ladd);
        right->addall(ladd);
        ladd = 0;
      }

      if(max_v < left->max_v) {
        left->update_max(max_v);
      }
      if(left->min_v < min_v) {
        left->update_min(min_v);
      }

      if(max_v < right->max_v) {
        right->update_max(max_v);
      }
      if(right->min_v < min_v) {
        right->update_min(min_v);
      }
    }

    void update() {
      sum = left->sum + right->sum;

      if(left->max_v < right->max_v) {
        max_v = right->max_v;
        max_c = right->max_c;
        smax_v = max(left->max_v, right->smax_v);
      } else if(left->max_v > right->max_v) {
        max_v = left->max_v;
        max_c = left->max_c;
        smax_v = max(left->smax_v, right->max_v);
      } else {
        max_v = left->max_v;
        max_c = left->max_c + right->max_c;
        smax_v = max(left->smax_v, right->smax_v);
      }

      if(left->min_v < right->min_v) {
        min_v = left->min_v;
        min_c = left->min_c;
        smin_v = min(left->smin_v, right->min_v);
      } else if(left->min_v > right->min_v) {
        min_v = right->min_v;
        min_c = right->min_c;
        smin_v = min(left->min_v, right->smin_v);
      } else {
        min_v = left->min_v;
        min_c = left->min_c + right->min_c;
        smin_v = min(left->smin_v, right->smin_v);
      }
    }
  };

  int n, n0;
  Node *root;

  void _update_min(ll x, int a, int b, Node *nd, int l, int r) {
    if(b <= l || r <= a || nd->max_v <= x) {
      return;
    }
    if(a <= l && r <= b && nd->smax_v < x) {
      nd->update_max(x);
      return;
    }

    nd->push();
    _update_min(x, a, b, nd->left, l, (l+r)/2);
    _update_min(x, a, b, nd->right, (l+r)/2, r);
    nd->update();
  }

  void _update_max(ll x, int a, int b, Node *nd, int l, int r) {
    if(b <= l || r <= a || x <= nd->min_v) {
      return;
    }
    if(a <= l && r <= b && x < nd->smin_v) {
      nd->update_min(x);
      return;
    }

    nd->push();
    _update_max(x, a, b, nd->left, l, (l+r)/2);
    _update_max(x, a, b, nd->right, (l+r)/2, r);
    nd->update();
  }

  void _add_val(ll x, int a, int b, Node *nd, int l, int r) {
    if(b <= l || r <= a) {
      return;
    }
    if(a <= l && r <= b) {
      nd->addall(x);
      return;
    }

    nd->push();
    _add_val(x, a, b, nd->left, l, (l+r)/2);
    _add_val(x, a, b, nd->right, (l+r)/2, r);
    nd->update();
  }

  void _update_val(ll x, int a, int b, Node *nd, int l, int r) {
    if(b <= l || r <= a) {
      return;
    }
    if(a <= l && r <= b) {
      nd->updateall(x);
      return;
    }

    nd->push();
    _update_val(x, a, b, nd->left, l, (l+r)/2);
    _update_val(x, a, b, nd->right, (l+r)/2, r);
    nd->update();
  }

  ll _query_max(int a, int b, Node *nd, int l, int r) {
    if(b <= l || r <= a) {
      return -inf;
    }
    if(a <= l && r <= b) {
      return nd->max_v;
    }
    nd->push();
    ll lv = _query_max(a, b, nd->left, l, (l+r)/2);
    ll rv = _query_max(a, b, nd->right, (l+r)/2, r);
    return max(lv, rv);
  }

  ll _query_min(int a, int b, Node *nd, int l, int r) {
    if(b <= l || r <= a) {
      return inf;
    }
    if(a <= l && r <= b) {
      return nd->min_v;
    }
    nd->push();
    ll lv = _query_min(a, b, nd->left, l, (l+r)/2);
    ll rv = _query_min(a, b, nd->right, (l+r)/2, r);
    return min(lv, rv);
  }

  ll _query_sum(int a, int b, Node *nd, int l, int r) {
    if(b <= l || r <= a) {
      return 0;
    }
    if(a <= l && r <= b) {
      return nd->sum;
    }
    nd->push();
    ll lv = _query_sum(a, b, nd->left, l, (l+r)/2);
    ll rv = _query_sum(a, b, nd->right, (l+r)/2, r);
    return lv + rv;
  }

public:
  SegmentTree(int n, ll *a) : n(n) {
    n0 = 1;
    while(n0 < n) n0 <<= 1;

    Node *nds = new Node[2*n0];
    root = nds;

    nds[0].len = n0;
    for(int i=0; i<n0-1; ++i) {
      nds[i].left = &nds[2*i+1];
      nds[i].right = &nds[2*i+2];
      nds[2*i+1].len = nds[2*i+2].len = (nds[i].len >> 1);
    }

    for(int i=0; i<n; ++i) nds[n0-1+i].init(a[i]);
    for(int i=n; i<n0; ++i) nds[n0-1+i].init_empty();
    for(int i=n0-2; i>=0; i--) nds[i].update();
  }

  void update_min(int a, int b, ll x) {
    _update_min(x, a, b, root, 0, n0);
  }

  void update_max(int a, int b, ll x) {
    _update_max(x, a, b, root, 0, n0);
  }

  void add_val(int a, int b, ll x) {
    _add_val(x, a, b, root, 0, n0);
  }

  void update_val(int a, int b, ll x) {
    _update_val(x, a, b, root, 0, n0);
  }

  ll query_max(int a, int b) {
    return _query_max(a, b, root, 0, n0);
  }

  ll query_min(int a, int b) {
    return _query_min(a, b, root, 0, n0);
  }

  ll query_sum(int a, int b) {
    return _query_sum(a, b, root, 0, n0);
  }
};

ll vv[N];
vector<tuple<int, int, int>> v;

signed main() {

    SS(int, NN, M);

    REP(i, 1, N) vv[i] = (ll)1e15 + 10;
    SegmentTree seg(N, vv);

    // seg.update_min(1, 2, 9);/
    // seg.update(0, 1, 0);
    // REP(i, 10) cerr << seg.query_min(i, i + 1) << endl;
    // return 0;

    REP(i, M) {
        SS(int, L, R, C);
        --L;
        v.emplace_back(L, R, C);
    }
    SORT(v);

    EACH(t, v) {
        int l, r, c;
        tie(l, r, c) = t;
        ll mn = seg.query_min(l, r);
        // debug(l, r, mn);
        seg.update_min(l, r, mn + c);
    }

    ll res = seg.query_min(NN - 1, NN);
    if (res >= (ll)(1e15)) res = -1;
    print(res);

    // REP(i, NN) cerr << seg.query_min(i, i + 1) << endl;

}

Measured memory_bytes: 27256