#pragma GCC optimize("O3,unroll-loops")
#pragma GCC target("avx,popcnt,sse4,abm")
#include <bits/stdc++.h>
using namespace std;
#ifndef WAIMAI
#include "fish.h"
#endif
#ifdef WAIMAI
#define debug(HEHE...) cout << "[" << #HEHE << "] : ", dout(HEHE)
void dout() {cout << '\n';}
template<typename T, typename...U>
void dout(T t, U...u) {cout << t << (sizeof...(u) ? ", " : ""), dout(u...);}
#else
#define debug(...) 7122
#endif
#define ll long long
#define Waimai ios::sync_with_stdio(false), cin.tie(0)
#define FOR(x,a,b) for (int x = a, I = b; x <= I; x++)
#define pb emplace_back
#define F first
#define S second
const ll INF = 1e18;
const int SIZE = 3005;
int n, m;
ll pre[SIZE][SIZE];
ll inc[2][SIZE], dek[2][SIZE], all[2], zero[2], fzero[2];
void chmax(ll &x, ll y) {
x = max(x, y);
}
ll max_weights(int N, int M, vector<int> X, vector<int> Y, vector<int> W) {
n = N;
FOR (i, 0, M - 1) {
X[i]++, Y[i]++;
pre[X[i]][Y[i]] += W[i];
m = max(m, Y[i]);
}
all[0] = -INF;
FOR (i, 1, m) inc[0][i] = dek[0][i] = -INF;
FOR (i, 1, n) {
FOR (j, 1, m) pre[i][j] += pre[i][j - 1];
int f = i & 1;
all[f] = zero[f] = fzero[f] = -INF;
FOR (j, 1, m) inc[f][j] = dek[f][j] = -INF;
FOR (j, 1, m) inc[!f][j] -= pre[i - 1][j];
ll mx = zero[!f];
FOR (j, 1, m) {
chmax(inc[f][j], mx + pre[i - 1][j]);
chmax(mx, inc[!f][j]);
}
FOR (j, 1, m) dek[!f][j] += pre[i][j];
mx = all[!f] + pre[i][m];
for (int j = m; j >= 0; j--) {
chmax(dek[f][j], mx - (j ? pre[i][j - 1] : 0));
chmax(mx, dek[!f][j]);
}
chmax(all[f], all[!f]);
chmax(all[f], fzero[!f]);
chmax(all[f], inc[f][m]);
chmax(zero[f], zero[!f]);
chmax(zero[f], fzero[!f]);
chmax(fzero[f], dek[!f][0]);
}
ll ans = 0;
int f = n & 1;
chmax(ans, zero[f]);
chmax(ans, fzero[f]);
chmax(ans, all[f]);
return ans;
}
/*
in1
5 4
0 2 5
1 1 2
4 4 1
3 3 3
out1
8
*/
#ifdef WAIMAI
int main() {
int N, M;
assert(2 == scanf("%d %d", &N, &M));
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
