Language: C++

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// need
#include <iostream>
#include <algorithm>
// data structure
#include <bitset>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <utility>
#include <vector>
#include <complex>
//#include <deque>
#include <valarray>
#include <unordered_map>
#include <unordered_set>
#include <array>
// etc
#include <cassert>
#include <cmath>
#include <functional>
#include <iomanip>
#include <chrono>
#include <random>
#include <numeric>
#include <fstream>

auto& scan_in = std::cin;

// input
#define INIT std::ios::sync_with_stdio(false);std::cin.tie(0);
#define VAR(type, ...)type __VA_ARGS__;MACRO_VAR_Scan(__VA_ARGS__);
template<typename T> void MACRO_VAR_Scan(T& t) { scan_in >> t; }
template<typename First, typename...Rest>void MACRO_VAR_Scan(First& first, Rest& ...rest) { scan_in >> first; MACRO_VAR_Scan(rest...); }
#define VEC_ROW(type, n, ...)std::vector<type> __VA_ARGS__;MACRO_VEC_ROW_Init(n, __VA_ARGS__); for(int w_=0; w_<n; ++w_){MACRO_VEC_ROW_Scan(w_, __VA_ARGS__);}
template<typename T> void MACRO_VEC_ROW_Init(int n, T& t) { t.resize(n); }
template<typename First, typename...Rest>void MACRO_VEC_ROW_Init(int n, First& first, Rest& ...rest) { first.resize(n); MACRO_VEC_ROW_Init(n, rest...); }
template<typename T> void MACRO_VEC_ROW_Scan(int p, T& t) { scan_in >> t[p]; }
template<typename First, typename...Rest>void MACRO_VEC_ROW_Scan(int p, First& first, Rest& ...rest) { scan_in >> first[p]; MACRO_VEC_ROW_Scan(p, rest...); }
#define VEC(type, c, n) std::vector<type> c(n);for(auto& i:c)scan_in>>i;
#define MAT(type, c, m, n) std::vector<std::vector<type>> c(m, std::vector<type>(n));for(auto& R:c)for(auto& w:R)scan_in>>w;
// output
template<typename T>void MACRO_OUT(const T t) { std::cout << t; }
template<typename First, typename...Rest>void MACRO_OUT(const First first, const Rest...rest) { std::cout << first << " "; MACRO_OUT(rest...); }
#define OUT(...) MACRO_OUT(__VA_ARGS__);
#define FOUT(n, dist) std::cout<<std::fixed<<std::setprecision(n)<<(dist);
#define SOUT(n, c, dist) std::cout<<std::setw(n)<<std::setfill(c)<<(dist);
#define VOUT(v) for(size_t i = 0; i < v.size(); ++i) {OUT(v[i]);if(i+1<v.size()){SP}}
#define EOUT(...) do{ OUT(__VA_ARGS__)BR; exit(0); }while(0);
#define SP std::cout<<" ";
#define TAB std::cout<<"\t";
#define BR std::cout<<"\n";
#define SPBR(w, n) std::cout<<(w + 1 == n ? '\n' : ' ');
#define ENDL std::cout<<std::endl;
#define FLUSH std::cout<<std::flush;
#define SHOW(dist) {std::cerr << #dist << "\t: " << (dist) << "\n";}
// utility
#define ALL(a) (a).begin(),(a).end()
#define FOR(w, a, n) for(int w=(a);w<(n);++w)
#define REP(w, n) FOR(w, 0, n)
#define RFOR(w, a, n) for(int w=(n)-1;w>=(a);--w)
#define RREP(w, n) RFOR(w, 0, n)
template<class S, class T, class U> bool IN(S a, T x, U b) { return a <= x && x < b; }
template<class T> inline T CHMAX(T& a, const T b) { return a = (a < b) ? b : a; }
template<class T> inline T CHMIN(T& a, const T b) { return a = (a > b) ? b : a; }
// test
template<class T> using V = std::vector<T>;
template<class T> using VV = V<V<T>>;

template<typename S, typename T>
std::ostream& operator<<(std::ostream& os, const std::pair<S, T>& p) {
	os << "(" << p.first << ", " << p.second << ")"; return os;
}
template<typename T>
std::ostream& operator<<(std::ostream& os, const std::vector<T>& v) {
	os << "{";
	for (size_t i = 0; i < v.size(); ++i) os << v[i] << ((i + 1 < v.size()) ? ", " : ""); os << "}";
	return os;
}
template<typename S, typename T>
std::ostream & operator<<(std::ostream & os, const std::map<S, T> & m) {
	os << "{";
	for (auto it = m.cbegin(); it != m.cend();) { os << it->first << ":" << it->second; ++it; os << (it == m.cend() ? "" : ", "); } os << "}";
	return os;
}
template<typename T>
std::ostream& operator<<(std::ostream & os, std::queue<T> q) {
	os << "<";
	while (!q.empty()) { os << q.front(); q.pop(); os << (q.empty() ? "<" : ", "); }
	return os;
}
template<typename T>
std::ostream& operator<<(std::ostream & os, std::stack<T> q) {
	os << ">";
	while (!q.empty()) { os << q.top(); q.pop(); os << (q.empty() ? "]" : ", "); }
	return os;
}
namespace std {
	template<typename S, typename T> class numeric_limits<pair<S, T>> {
	public:
		static constexpr pair<S, T> max() noexcept { return { numeric_limits<S>::max(), numeric_limits<T>::max() }; }
		static constexpr pair<S, T> lowest() noexcept { return { numeric_limits<S>::lowest(), numeric_limits<T>::lowest() }; }
	};
}

// type/const
using i64 = std::int_fast64_t;
using u64 = std::uint_fast64_t;
using ll = long long;
using ull = unsigned long long;
using ld = long double;
using PAIR = std::pair<i64, i64>;
using PAIRLL = std::pair<ll, ll>;
constexpr int INFINT = (1 << 30) - 1;                    // 1.07x10^ 9
constexpr int INFINT_LIM = (1LL << 31) - 1;              // 2.15x10^ 9
constexpr ll INFLL = 1LL << 60;                          // 1.15x10^18
constexpr ll INFLL_LIM = (1LL << 62) - 1 + (1LL << 62);  // 9.22x10^18
constexpr double EPS = 1e-6;
constexpr i64 MOD = 1000000007;
constexpr double PI = 3.141592653589793238462643383279;
template<class T, size_t N> void FILL(T(&a)[N], const T & val) { for (auto& x : a) x = val; }
template<class ARY, size_t N, size_t M, class T> void FILL(ARY(&a)[N][M], const T & val) { for (auto& b : a) FILL(b, val); }
template<class T> void FILL(std::vector<T> & a, const T & val) { for (auto& x : a) x = val; }
template<class ARY, class T> void FILL(std::vector<std::vector<ARY>> & a, const T & val) { for (auto& b : a) FILL(b, val); }
// ------------>8---------

i64 dp[3003][3003][4];
i64 item[3003][3003];

signed main() {
	INIT;

	VAR(i64, r, c, k);
	REP(_, k) {
		VAR(i64, y, x, v);
		item[y][x] = v;
	}

	FILL(dp, -INFLL);
	dp[0][0][0] = 0;

	REP(i, r + 1) REP(j, c + 1) REP(p, 4) {
		if (dp[i][j][p] == -INFLL) continue;
		if (i < r) {
			if (item[i + 1][j] > 0) {
				CHMAX(dp[i + 1][j][1], dp[i][j][p] + item[i + 1][j]);
			}
			CHMAX(dp[i + 1][j][0], dp[i][j][p]);
		}
		if (j < c) {
			if (item[i][j + 1] > 0) {
				if (p < 3) CHMAX(dp[i][j + 1][p + 1], dp[i][j][p] + item[i][j + 1]);
			}
			CHMAX(dp[i][j + 1][p], dp[i][j][p]);
		}
	}
	i64 ans = 0;
	REP(p, 4) CHMAX(ans, dp[r][c][p]);
	OUT(ans)BR;

	return 0;
}

Compiler output:
a.cc: In instantiation of 'void FILL(ARY (&)[N][M], const T&) [with ARY = long int; long unsigned int N = 3003; long unsigned int M = 4; T = long long int]':
a.cc:163:114:   required from 'void FILL(ARY (&)[N][M], const T&) [with ARY = long int [4]; long unsigned int N = 3003; long unsigned int M = 3003; T = long long int]'
  163 | template<class ARY, size_t N, size_t M, class T> void FILL(ARY(&a)[N][M], const T & val) { for (auto& b : a) FILL(b, val); }
      |                                                                                                              ~~~~^~~~~~~~
a.cc:180:6:   required from here
  180 |         FILL(dp, -INFLL);
      |         ~~~~^~~~~~~~~~~~
a.cc:163:114: error: no matching function for call to 'FILL(long int [4], const long long int&)'
  163 | template<class ARY, size_t N, size_t M, class T> void FILL(ARY(&a)[N][M], const T & val) { for (auto& b : a) FILL(b, val); }
      |                                                                                                              ~~~~^~~~~~~~
a.cc:162:34: note: candidate: 'template<class T, long unsigned int N> void FILL(T (&)[N], const T&)'
  162 | template<class T, size_t N> void FILL(T(&a)[N], const T & val) { for (auto& x : a) x = val; }
      |                                  ^~~~
a.cc:162:34: note:   template argument deduction/substitution failed:
a.cc:163:114: note:   deduced conflicting types for parameter 'const T' ('long int' and 'long long int')
  163 | template<class ARY, size_t N, size_t M, class T> void FILL(ARY(&a)[N][M], const T & val) { for (auto& b : a) FILL(b, val); }
      |                                                                                                              ~~~~^~~~~~~~
a.cc:163:55: note: candidate: 'template<class ARY, long unsigned int N, long unsigned int M, class T> void FILL(ARY (&)[N][M], const T&)'
  163 | template<class ARY, size_t N, size_t M, class T> void FILL(ARY(&a)[N][M], const T & val) { for (auto& b : a) FILL(b, val); }
      |                                                       ^~~~
a.cc:163:55: note:   template argument deduction/substitution failed:
a.cc:163:114: note:   mismatched types 'ARY [M]' and 'long int'
  163 | template<class ARY, size_t N, size_t M, class T> void FILL(ARY(&a)[N][M], const T & val) { for (auto& b : a) FILL(b, val); }
      |                                                                                                              ~~~~^~~~~~~~