import std.conv, std.functional, std.range, std.stdio, std.string;
import std.algorithm, std.array, std.bigint, std.bitmanip, std.complex, std.container, std.math, std.numeric, std.regex, std.typecons;
import core.bitop;

class EOFException : Throwable { this() { super("EOF"); } }
string[] tokens;
string readToken() { for (; tokens.empty; ) { if (stdin.eof) { throw new EOFException; } tokens = readln.split; } auto token = tokens.front; tokens.popFront; return token; }
int readInt() { return readToken.to!int; }
long readLong() { return readToken.to!long; }
real readReal() { return readToken.to!real; }

bool chmin(T)(ref T t, in T f) { if (t > f) { t = f; return true; } else { return false; } }
bool chmax(T)(ref T t, in T f) { if (t < f) { t = f; return true; } else { return false; } }

int binarySearch(alias pred, T)(in T[] as) { int lo = -1, hi = cast(int)(as.length); for (; lo + 1 < hi; ) { const mid = (lo + hi) >> 1; (unaryFun!pred(as[mid]) ? hi : lo) = mid; } return hi; }
int lowerBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a >= val)); }
int upperBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a > val)); }


enum INF = 10L^^18;

void bChmin(T)(T[] bit, int pos, T val) {
  for (int x = pos; x < bit.length; x |= x + 1) {
    chmin(bit[x], val);
  }
}

// min of [0, pos)
T bMin(T)(T[] bit, int pos) {
  T mn = INF;
  for (int x = pos - 1; x >= 0; x = (x & (x + 1)) - 1) {
    chmin(mn, bit[x]);
  }
  return mn;
}

alias Interval = Tuple!(int, "a", int, "b", long, "c");

void main() {
  try {
    for (; ; ) {
      const N = readInt();
      const M = readInt();
      auto P = new Interval[M];
      foreach (i; 0 .. M) {
        P[i].a = readInt();
        P[i].b = readInt();
        P[i].c = readLong();
      }
      
      P.sort!((p, q) => (p.b < q.b));
      debug {
        writeln("P = ", P);
      }
      
      auto bit = new long[N + 1];
      bit[] = INF;
      
      bit.bChmin(N - 1, 0);
      foreach (i; 0 .. M) {
        // j s.t. A[i] <= B[j]
        const res = bit.bMin(N - P[i].a + 1);
        debug {
          writeln(i, ": ", res + P[i].c);
        }
        bit.bChmin(N - P[i].b, res + P[i].c);
      }
      // j s.t. N <= B[j]
      const ans = bit.bMin(N - N + 1);
      writeln((ans >= INF) ? -1 : ans);
    }
  } catch (EOFException e) {
  }
}
