#include <cstdio>
#include <cctype>
#include <cstring>
#include <algorithm>
#include <numeric>
#include <vector>
#include <cassert>
#include "fish.h"
using namespace std;

using ll = long long;
using ull = unsigned long long;

#define LOG(f...) fprintf(stderr, f)
#define DBG(f...) printf("%3d: ", __LINE__), printf(f)
// #define DBG(f...) void()
#define all(cont) begin(cont), end(cont)

const int INF = 0x3f3f3f3f;

struct fish {
  int y, w;
};

ll max_weights(int n, int m, vector<int> x, vector<int> y, vector<int> w) {
  vector<vector<fish>> v(n + 1);
  for (int i = 0; i < m; ++i)
    v[x[i]].push_back({y[i], w[i]});
  for (int i = 0; i < n; ++i) {
    sort(all(v[i]), [](fish a, fish b) { return a.y < b.y; });
    v[i].push_back({INF, 0});
  }
  ll mx1 = 0, mx2 = 0, ans = 0;
  ll pre = 0, pre2 = 0, pre3 = 0;
  for (int i = 0; i < n; ++i) {
    ll s0 = 0, s1 = 0, s2 = 0;
    const auto &L = (i == 0 ? v[n] : v[i - 1]);
    const auto &R = v[i + 1];
    ll _mx1 = 0, _mx2 = 0;
    pre3 = pre2; pre2 = pre; pre = 0;
    auto itl = L.begin(), itr = R.begin();
    for (fish f : v[i]) {
      while (itl != L.end() && itl->y < f.y)
        s0 += itl->w, ++itl;
      while (itr != R.end() && itr->y < f.y)
        s2 += itr->w, ++itr;
      ll valU = max(s0 + mx1, pre3);
      ll valD = mx2 - s1;
      // DBG("i = %d y = %d valU = %lld valD = %lld\n", i, f.y, valU, valD);
      ans = max(ans, valU); ans = max(ans, valD);
      _mx1 = max(_mx1, valU - s1);
      _mx2 = max(_mx2, max(valU, valD) + s2);
      pre = max(pre, max(valU, valD) + s2);
      // _mx1 = max(_mx1, val - s1);
      // _mx2 = max(_mx2, val + s2);
      s1 += f.w;
    }
    mx1 = _mx1; mx2 = _mx2;
  }
  return ans;
}

#ifdef LOCAL
#include "fish.h"

#include <cassert>
#include <cstdio>

#include <vector>

int main() {
  freopen("input.txt", "r", stdin);
  freopen("output.txt", "w", stdout);
  int N, M;
  assert(2 == scanf("%d %d", &N, &M));

  std::vector<int> X(M), Y(M), W(M);
  for (int i = 0; i < M; ++i) {
    assert(3 == scanf("%d %d %d", &X[i], &Y[i], &W[i]));
  }

  long long result = max_weights(N, M, X, Y, W);
  printf("%lld\n", result);
  return 0;
}

#endif