#include <bits/stdc++.h>
using namespace std;
const long long INF = 1000000000000000000;
int main(){
  int N, M;
  cin >> N >> M;
  vector<vector<int>> A(N, vector<int>(N, 0));
  for (int i = 0; i < M; i++){
    int h, w, c;
    cin >> h >> w >> c;
    h--;
    w--;
    A[h][w] = c;
  }
  vector<vector<vector<long long>>> dp(N, vector<vector<long long>>(N, vector<long long>(2, INF)));
  dp[0][0][0] = 0;
  for (int i = 0; i < N; i++){
    for (int j = 0; j < N; j++){
      if (i < N - 1){
        dp[i + 1][j][0] = min(dp[i + 1][j][0], dp[i][j][0] + A[i + 1][j] + 1);
        dp[i + 1][j][1] = min(dp[i + 1][j][1], dp[i][j][1] + A[i + 1][j] + 1);
        dp[i + 1][j][1] = min(dp[i + 1][j][1], dp[i][j][0] + 1);
      }
      if (j < N - 1){
        dp[i][j + 1][0] = min(dp[i][j + 1][0], dp[i][j][0] + A[i][j + 1] + 1);
        dp[i][j + 1][1] = min(dp[i][j + 1][1], dp[i][j][1] + A[i][j + 1] + 1);
        dp[i][j + 1][1] = min(dp[i][j + 1][1], dp[i][j][0] + 1);
      }
    }
  }
  cout << dp[N - 1][N - 1][1] << endl;
}