import std.stdio, std.conv, std.functional, std.string;
import std.algorithm, std.array, std.container, std.range, std.typecons;
import std.bigint, 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; }

void chmin(T)(ref T x, T y) { x = min(x, y); } void chmax(T)(ref T x, T y) { x = max(x, y); }
bool inside(T)(T x, T b, T e) { return x >= b && x < e; }
T lcm(T)(T x, T y) { return x * (y / gcd(x, y)); }

//long mod = 10^^9 + 7;
long mod = 998_244_353;
//long mod = 1_000_003;
void moda(T)(ref T x, T y) { x = (x + y) % mod; }
void mods(T)(ref T x, T y) { x = ((x + mod) - (y % mod)) % mod; }
void modm(T)(ref T x, T y) { x = (x * y) % mod; }
void modpow(T)(ref T x, T y) { if (!y) { x = 1; return; } auto t = x; x.modpow(y>>1); x.modm(x); if (y&1) x.modm(t); }
void modd(T)(ref T x, T y) { y.modpow(mod - 2); x.modm(y); }

void main()
{
	auto N = RD;
	auto M = RD;
	auto cost = new long[][](N, N);
	foreach (i; 0..M)
	{
		auto h = RD-1;
		auto w = RD-1;
		auto c = RD;
		cost[h][w] = c;
	}

	auto dist = new long[][][](N, N, 2);
	foreach (y; 0..N)
	{
		foreach (x; 0..N)
		{
			dist[y][x][] = long.max;
		}
	}
	dist[0][0][] = 0;

	BinaryHeap!(Array!(long[]), "a[3] > b[3]") q;
	q.insert([0, 0, 0, 0]);
	while (!q.empty)
	{
		auto nd = q.front; q.removeFront;
		auto y = nd[0];
		auto x = nd[1];
		auto s = nd[2];
		auto c = nd[3];
		foreach (dir; [[-1, 0], [1, 0], [0, -1], [0, 1]])
		{
			auto ny = y + dir[0];
			auto nx = x + dir[1];
			if (inside(ny, 0, N) && inside(nx, 0, N))
			{
				auto nc = c + 1 + cost[y][x];
				if (nc < dist[ny][nx][s])
				{
					dist[ny][nx][s] = nc;
					q.insert([ny, nx, s, nc]);
				}
				if (s == 0)
				{
					if (c+1 < dist[ny][nx][1])
					{
						dist[ny][nx][1] = c+1;
						q.insert([ny, nx, 1, c+1]);
					}
				}
			}
		}
	}
	auto ans = min(dist[N-1][N-1][0], dist[N-1][N-1][1]);

	writeln(ans);
	stdout.flush;
	debug readln;
}