#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define pb(x) push_back(x)
#define mp(a, b) make_pair(a, b)
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
#define lscan(x) scanf("%I64d", &x)
#define lprint(x) printf("%I64d", x)
#define rep(i, n) for (ll i = 0; i < (n); i++)
#define rep2(i, n) for (ll i = n - 1; i >= 0; i--)
template <class T>
using rque = priority_queue<T, vector<T>, greater<T>>;
const ll mod = 1e9+7;

ll gcd(ll a, ll b)
{
	ll c = a % b;
	while (c != 0)
	{
		a = b;
		b = c;
		c = a % b;
	}
	return b;
}

long long extGCD(long long a, long long b, long long &x, long long &y)
{
	if (b == 0)
	{
		x = 1;
		y = 0;
		return a;
	}
	long long d = extGCD(b, a % b, y, x);
	y -= a / b * x;
	return d;
}

struct UnionFind
{
	vector<ll> data;

	UnionFind(int sz)
	{
		data.assign(sz, -1);
	}

	bool unite(int x, int y)
	{
		x = find(x), y = find(y);
		if (x == y)
			return (false);
		if (data[x] > data[y])
			swap(x, y);
		data[x] += data[y];
		data[y] = x;
		return (true);
	}

	int find(int k)
	{
		if (data[k] < 0)
			return (k);
		return (data[k] = find(data[k]));
	}

	ll size(int k)
	{
		return (-data[find(k)]);
	}
};

ll M = 1000000007;

vector<ll> fac(2000011);  //n!(mod M)
vector<ll> ifac(2000011); //k!^{M-2} (mod M)

ll mpow(ll x, ll n)
{
	ll ans = 1;
	while (n != 0)
	{
		if (n & 1)
			ans = ans * x % M;
		x = x * x % M;
		n = n >> 1;
	}
	return ans;
}
ll mpow2(ll x, ll n, ll mod)
{
	ll ans = 1;
	while (n != 0)
	{
		if (n & 1)
			ans = ans * x % mod;
		x = x * x % mod;
		n = n >> 1;
	}
	return ans;
}
void setcomb()
{
	fac[0] = 1;
	ifac[0] = 1;
	for (ll i = 0; i < 2000010; i++)
	{
		fac[i + 1] = fac[i] * (i + 1) % M; // n!(mod M)
	}
	ifac[2000010] = mpow(fac[2000010], M - 2);
	for (ll i = 2000010; i > 0; i--)
	{
		ifac[i - 1] = ifac[i] * i % M;
	}
}
ll comb(ll a, ll b)
{
	if (a == 0 && b == 0)
		return 1;
	if (a < b || a < 0)
		return 0;
	ll tmp = ifac[a - b] * ifac[b] % M;
	return tmp * fac[a] % M;
}
ll perm(ll a, ll b)
{
	if (a == 0 && b == 0)
		return 1;
	if (a < b || a < 0)
		return 0;
	return fac[a] * ifac[a - b] % M;
}
long long modinv(long long a)
{
	long long b = M, u = 1, v = 0;
	while (b)
	{
		long long t = a / b;
		a -= t * b;
		swap(a, b);
		u -= t * v;
		swap(u, v);
	}
	u %= M;
	if (u < 0)
		u += M;
	return u;
}
ll modinv2(ll a, ll mod)
{
	ll b = mod, u = 1, v = 0;
	while (b)
	{
		ll t = a / b;
		a -= t * b;
		swap(a, b);
		u -= t * v;
		swap(u, v);
	}
	u %= mod;
	if (u < 0)
		u += mod;
	return u;
}

template <int mod>
struct ModInt
{
	int x;

	ModInt() : x(0) {}

	ModInt(int64_t y) : x(y >= 0 ? y % mod : (mod - (-y) % mod) % mod) {}

	ModInt &operator+=(const ModInt &p)
	{
		if ((x += p.x) >= mod)
			x -= mod;
		return *this;
	}

	ModInt &operator-=(const ModInt &p)
	{
		if ((x += mod - p.x) >= mod)
			x -= mod;
		return *this;
	}

	ModInt &operator*=(const ModInt &p)
	{
		x = (int)(1LL * x * p.x % mod);
		return *this;
	}

	ModInt &operator/=(const ModInt &p)
	{
		*this *= p.inverse();
		return *this;
	}

	ModInt operator-() const { return ModInt(-x); }

	ModInt operator+(const ModInt &p) const { return ModInt(*this) += p; }

	ModInt operator-(const ModInt &p) const { return ModInt(*this) -= p; }

	ModInt operator*(const ModInt &p) const { return ModInt(*this) *= p; }

	ModInt operator/(const ModInt &p) const { return ModInt(*this) /= p; }

	bool operator==(const ModInt &p) const { return x == p.x; }

	bool operator!=(const ModInt &p) const { return x != p.x; }

	ModInt inverse() const
	{
		int a = x, b = mod, u = 1, v = 0, t;
		while (b > 0)
		{
			t = a / b;
			swap(a -= t * b, b);
			swap(u -= t * v, v);
		}
		return ModInt(u);
	}

	ModInt pow(int64_t n) const
	{
		ModInt ret(1), mul(x);
		while (n > 0)
		{
			if (n & 1)
				ret *= mul;
			mul *= mul;
			n >>= 1;
		}
		return ret;
	}

	friend ostream &operator<<(ostream &os, const ModInt &p)
	{
		return os << p.x;
	}

	friend istream &operator>>(istream &is, ModInt &a)
	{
		int64_t t;
		is >> t;
		a = ModInt<mod>(t);
		return (is);
	}

	static int get_mod() { return mod; }
};

using mint = ModInt<mod>;

typedef vector<vector<mint>> Matrix;

Matrix mul(Matrix a, Matrix b)
{
	int i, j, k;
	mint t;
	int n = a.size(), m = b[0].size(), l = a[0].size();
	Matrix c(n, vector<mint>(m));
	for (i = 0; i < n; i++)
	{
		for (j = 0; j < m; j++)
		{
			t = 0;
			for (k = 0; k < l; k++)
				t += a[i][k] * b[k][j];
			c[i][j] = t;
		}
	}
	return c;
}

Matrix mat_pow(Matrix x, ll n)
{
	ll k = x.size();
	Matrix ans(k, vector<mint>(k, 0));
	for (int i = 0; i < k; i++)
		ans[i][i] = 1;
	while (n != 0)
	{
		if (n & 1)
			ans = mul(ans, x);
		x = mul(x, x);
		n = n >> 1;
	}
	return ans;
}

template <typename Monoid>
struct SegmentTree
{
    using F = function<Monoid(Monoid, Monoid)>;

    int sz;
    vector<Monoid> seg;

    const F f;
    const Monoid M1;

    SegmentTree(int n, const F f, const Monoid &M1) : f(f), M1(M1)
    {
        sz = 1;
        while (sz < n)
            sz <<= 1;
        seg.assign(2 * sz, M1);
    }

    void set(int k, const Monoid &x)
    {
        seg[k + sz] = x;
    }

    void build()
    {
        for (int k = sz - 1; k > 0; k--)
        {
            seg[k] = f(seg[2 * k + 0], seg[2 * k + 1]);
        }
    }

    void update(int k, const Monoid &x)
    {
        k += sz;
        seg[k] = x;
        while (k >>= 1)
        {
            seg[k] = f(seg[2 * k + 0], seg[2 * k + 1]);
        }
    }

    Monoid query(int a, int b)
    {
        Monoid L = M1, R = M1;
        for (a += sz, b += sz; a < b; a >>= 1, b >>= 1)
        {
            if (a & 1)
                L = f(L, seg[a++]);
            if (b & 1)
                R = f(seg[--b], R);
        }
        return f(L, R);
    }

    Monoid operator[](const int &k) const
    {
        return seg[k + sz];
    }

    template <typename C>
    int find_subtree(int a, const C &check, Monoid &M, bool type)
    {
        while (a < sz)
        {
            Monoid nxt = type ? f(seg[2 * a + type], M) : f(M, seg[2 * a + type]);
            if (check(nxt))
                a = 2 * a + type;
            else
                M = nxt, a = 2 * a + 1 - type;
        }
        return a - sz;
    }

    template <typename C>
    int find_first(int a, const C &check)
    {
        Monoid L = M1;
        if (a <= 0)
        {
            if (check(f(L, seg[1])))
                return find_subtree(1, check, L, false);
            return -1;
        }
        int b = sz;
        for (a += sz, b += sz; a < b; a >>= 1, b >>= 1)
        {
            if (a & 1)
            {
                Monoid nxt = f(L, seg[a]);
                if (check(nxt))
                    return find_subtree(a, check, L, false);
                L = nxt;
                ++a;
            }
        }
        return -1;
    }

    template <typename C>
    int find_last(int b, const C &check)
    {
        Monoid R = M1;
        if (b >= sz)
        {
            if (check(f(seg[1], R)))
                return find_subtree(1, check, R, true);
            return -1;
        }
        int a = sz;
        for (b += sz; a < b; a >>= 1, b >>= 1)
        {
            if (b & 1)
            {
                Monoid nxt = f(seg[--b], R);
                if (check(nxt))
                    return find_subtree(b, check, R, true);
                R = nxt;
            }
        }
        return -1;
    }
};

int main()
{
	ll n, m;
	cin >> n >> m;
	ll h, w, c, cost[n][n] = {};
	rep(i, m) cin >> h >> w >> c, cost[--h][--w] = c;
	rque<pair<ll, pair<pair<ll, ll>, bool>>> que;
	ll d[n][n][2];
	rep(i, n) rep(j, n) rep(k, 2) d[i][j][k] = 1e15;
	que.push(mp(0, mp(mp(0, 0), false)));
	while(!que.empty()){
		auto q = que.top();
		que.pop();
		ll nd = q.first, nh = q.second.first.first, nw = q.second.first.second;
		bool used = q.second.second;
		if(nd < d[nh][nw][used]){
			if(nh == n-1 && nw == n-1){
				cout << nd << endl;
				return 0;
			}
			d[nh][nw][used] = nd;
			if(nh > 0){
				if(!used)
					que.push(mp(nd + 1, mp(mp(nh - 1, nw), true)));
				que.push(mp(nd + 1 + cost[nh-1][nw], mp(mp(nh - 1, nw), used)));
			}
			if(nh+1<n){
				if(!used)
					que.push(mp(nd + 1, mp(mp(nh + 1, nw), true)));
				que.push(mp(nd + 1 + cost[nh+1][nw], mp(mp(nh + 1, nw), used)));
			}
			if(nw > 0){
				if(!used)
					que.push(mp(nd + 1, mp(mp(nh, nw - 1), true)));
				que.push(mp(nd + 1 + cost[nh][nw - 1], mp(mp(nh, nw - 1), used)));
			}
			if(nw+1<n){
				if(!used)
					que.push(mp(nd + 1, mp(mp(nh, nw + 1), true)));
				que.push(mp(nd + 1 + cost[nh][nw + 1], mp(mp(nh, nw + 1), used)));
			}
		}
	}
}