#include <bits/stdc++.h>
#define rep(i,n) for(int i=0;i<(int)(n);i++)
using namespace std;
using ll = long long ;
using P = pair<int,int> ;
using pll = pair<long long,long long>;
constexpr int INF = 1e9;
constexpr long long LINF = 1e17;
constexpr int MOD = 1000000007;
constexpr double PI = 3.14159265358979323846;

ll dp[505][505][2];
ll check[505][505];

int main(){
    int n,m;
    cin >> n >> m;
    rep(i,m){
        int h,w;
        ll c;
        cin >> h >> w >> c;
        --h;--w;
        check[h][w] = c;
    }
    rep(i,n)rep(j,n)rep(k,2) dp[i][j][k] = LINF;
    dp[0][0][0] = 0;
    rep(i,n)rep(j,n){
        if(i ==0 && j == 0) continue;
        if(check[i][j] == 0){
            if(i > 0){
                dp[i][j][0] = min(dp[i][j][0], dp[i-1][j][0] + 1);
                dp[i][j][1] = min(dp[i][j][1], dp[i-1][j][1] + 1);
            }
            if(j > 0){
                dp[i][j][0] = min(dp[i][j][0], dp[i][j-1][0] + 1);
                dp[i][j][1] = min(dp[i][j][1], dp[i][j-1][1] + 1);
            }
        }else{
            if(i > 0){
                dp[i][j][0] = min(dp[i][j][0], dp[i-1][j][0] + 1 + check[i][j]);
                dp[i][j][1] = min(dp[i][j][1], dp[i-1][j][1] + 1 + check[i][j]);
                dp[i][j][1] = min(dp[i][j][1], dp[i-1][j][0] + 1);
            }
            if(j > 0){
                dp[i][j][0] = min(dp[i][j][0], dp[i][j-1][0] + 1 + check[i][j]);
                dp[i][j][1] = min(dp[i][j][1], dp[i][j-1][1] + 1 + check[i][j]);
                dp[i][j][1] = min(dp[i][j][1], dp[i][j-1][0] + 1);
            }
        }
    }
    cout << min(dp[n-1][n-1][0],dp[n-1][n-1][1]) << endl;
    return 0;
}