#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 = 1e5 + 5;
int n, m, R;
vector<pair<int, int>> v[SIZE];
ll all[2], zero[2], fzero[2];
map<int, ll> inc[2], dek[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]++;
v[X[i]].pb(Y[i], W[i]);
m = max(m, Y[i]);
R = max(R, X[i] + 1);
}
R = min(R, n);
debug(R);
all[0] = -INF;
FOR (i, 1, R) {
int f = i & 1;
inc[f].clear();
dek[f].clear();
all[f] = zero[f] = fzero[f] = -INF;
map<int, int> mp;
mp[0] = mp[m] = 0;
for (auto [j, x] : v[i]) mp[j] = 0;
for (auto [j, x] : v[i - 1]) mp[j] = x;
ll mx = zero[!f], sum = 0;
auto it = inc[!f].begin();
for (auto [j, x] : mp) {
while (it != inc[!f].end() && it->F <= j) chmax(mx, it->S - sum), it++;
sum += x;
// debug(j, x, sum, mx);
if (inc[f].count(j)) chmax(inc[f][j], mx + sum);
else inc[f][j] = mx + sum;
}
mp.clear();
mp[0] = mp[m] = 0;
sum = 0;
for (auto [j, x] : v[i - 1]) mp[j] = 0;
for (auto [j, x] : v[i]) mp[j] = x, sum += x;
mx = -INF;
auto rit = dek[!f].rbegin();
for (auto mit = mp.rbegin(); mit != mp.rend(); mit++) {
auto [j, x] = *mit;
while (rit != dek[!f].rend() && rit->F >= j) chmax(mx, rit->S + sum), rit++;
if (dek[f].count(j)) chmax(dek[f][j], mx - sum);
else dek[f][j] = mx - sum;
sum -= x;
}
reverse(v[i].begin(), v[i].end());
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]);
chmax(dek[f][m], all[f]);
for (auto it = inc[f].begin(); it != inc[f].end();) {
if (it != inc[f].begin() && it->S == prev(it)->S) it = inc[f].erase(it);
else it++;
}
vector<pair<int, ll>> tmp;
for (auto it = dek[f].rbegin(); it != dek[f].rend();) {
auto [j, x] = *it;
it++;
if (tmp.size() && x == tmp.back().S) continue;
else tmp.pb(j, x);
}
dek[f].clear();
for (auto [j, x] : tmp) dek[f][j] = x;
// debug(i);
// for (auto [j, x] : inc[f]) debug("inc", j, x);
// for (auto [j, x] : dek[f]) debug("dek", j, x);
}
ll ans = 0;
int f = R & 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
