#include <bits/stdc++.h>
using namespace std;
#ifdef LOCAL_
void debug_out() {cerr << endl;}
template<typename Head,typename... Tail> void debug_out(Head H,Tail... T){cerr << ' ' << H; debug_out(T...);}
#define debug(...) cerr << 'L' << __LINE__ << " [" << #__VA_ARGS__ << "]:",debug_out(__VA_ARGS__)
#define dump(x) cerr << 'L' << __LINE__ << " " << #x << " = " << (x) << endl;
#else
#define debug(...) (void(0))
#define dump(x) (void(0))
#endif
#define rep(i,n) for (int i = 0; i < (int)(n); i ++)
#define irep(i,n) for (int i = (int)(n) - 1;i >= 0;--i)
using ll = long long;
using PL = pair<ll,ll>;
using P = pair<int,int>;
constexpr int INF = 1000000000;
constexpr long long HINF = 1000000000000000;
constexpr long long MOD = 1000000007;// = 998244353;
constexpr double EPS = 1e-4;
constexpr double PI = 3.14159265358979;

int main() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);

    int N,M; cin >> N >> M;
    vector<vector<ll>> penalty(N,vector<ll>(N,0));
    rep(i,M) {
        int h,w,c; cin >> h >> w >> c;
        --h; --w;
        penalty[h][w] = c;
    }

    vector<vector<ll>> dp0(N,vector<ll>(N,HINF));
    vector<vector<ll>> dp1(N,vector<ll>(N,HINF));

    dp0[0][0] = 0;
    dp1[0][0] = 0;
    rep(i,N) {
        rep(j,N) {
            if (i + 1 < N) {
                dp0[i+1][j] = min(dp0[i+1][j],dp0[i][j]+1LL+penalty[i+1][j]);
                dp1[i+1][j] = min({dp1[i+1][j],dp0[i][j]+1LL,dp1[i][j]+1+penalty[i+1][j]});
            }
            if (j + 1 < N) {
                dp0[i][j+1] = min(dp0[i][j+1],dp0[i][j]+1LL+penalty[i][j+1]);
                dp1[i][j+1] = min({dp1[i][j+1],dp0[i][j]+1LL,dp1[i][j]+1+penalty[i][j+1]});
            }
        }
    }
    cout << dp1[N-1][N-1] << '\n';
    return 0;
}