import std.stdio, std.conv, std.functional, std.string;
import std.algorithm, std.array, std.container, std.range, std.typecons;
import std.numeric, std.math, std.random;
import core.bitop;

string FMT_F = "%.10f";

static File _f;
void file_io(string fn) { _f = File(fn, "r"); }
static string[] s_rd;
T _RD(T = long)() { while(!s_rd.length) s_rd = readln.chomp.split; string res = s_rd[0]; s_rd.popFront; return res.to!T; }
T _RD(T = long)(File f) { while(!s_rd.length) s_rd = f.readln.chomp.split; string res = s_rd[0]; s_rd.popFront; return res.to!T; }
T[] _RDA(T = long)(T fix = 0) { auto r = readln.chomp.split.to!(T[]); r[] += fix; return r; }
T[] _RDA(T = long)(File f, T fix = 0) { auto r = f.readln.chomp.split.to!(T[]); r[] += fix; return r; }
T RD(T = long)() { if (_f.isOpen) return _RD!T(_f); else return _RD!T; }
T[] RDA(T = long)(T fix = 0) { if (_f.isOpen)  return _RDA!T(_f, fix); else return _RDA!T(fix); }

size_t[] MAKE_IDX(alias less = "a < b", Range)(Range range) { auto idx = new size_t[](range.length); makeIndex!(less)(range, idx); return idx;}
size_t MIN_POS(alias less = "a < b", Range)(Range range) { auto r = minPos!(less)(range); return range.length - r.length; }

bool inside(T)(T x, T b, T e) { return x >= b && x < e; }
long lcm(long x, long y) { return x * (y / gcd(x, y)); }

//long mod = 10^^9 + 7;
long mod = 998244353;
//long mod = 1_000_003;
void moda(ref long x, long y) { x = (x + y) % mod; }
void mods(ref long x, long y) { x = ((x + mod) - (y % mod)) % mod; }
void modm(ref long x, long y) { x = (x * y) % mod; }

int binarySearch(alias pred, T)(in T[] arr, bool isLower = true)
{ 
	int ok, ng;
	if (isLower) { ok = cast(int)arr.length; ng = -1; }
	else		 { ok = -1; ng = cast(int)arr.length; }
	while (abs(ok-ng) > 1)
	{
		auto mid = (ok+ng) / 2;
		if (unaryFun!pred(arr[mid]))
			ok = mid;
		else ng = mid;
	}
	return ok;
}

void main()
{
	auto N = RD;
	auto M = RD;
	auto LRC = new long[][](M, 3);
	foreach (i; 0..M)
	{
		auto l = RD-1;
		auto r = RD-1;
		auto c = RD;
		LRC[i] = [l, r, c];
	}
	LRC.sort!"a[0] < b[0]"();
	debug writeln(LRC);

	auto dp = new long[2][](N);
	foreach (i; 0..N)
		dp[i] = [long.max, 0];
	dp[0] = [0, 0];
	foreach (i; 0..N)
	{
		if (dp[i][0] == long.max) continue;
		auto l = dp[i][1];
		auto r = LRC.binarySearch!((in long[] a) => a[0] > i)();
		debug writeln("i:", i, " lr:", l, ", ", r);
		foreach (j; l..r)
		{
			auto pos = LRC[j][1];
			if (dp[i][0]+LRC[j][2] < dp[pos][0])
			{
				dp[pos] = [dp[i][0]+LRC[j][2], j+1];
			}
		}
	}
	debug writeln(dp);

	writeln(dp[N-1][0] == long.max ? -1 : dp[N-1][0]);
	stdout.flush();
	debug readln();
}