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#include <iostream>
#include <vector>
#include <chrono>
#include <unordered_map>
#include <tuple>

using namespace std;

struct tuple_hash {
	template <class T>
	inline void hash_combine(std::size_t& seed, const T& val) const {
		seed ^= std::hash<T>{}(val)+0x9e3779b9 + (seed << 6) + (seed >> 2);
	}

	size_t operator()(const tuple<int, int, int>& t) const {
		size_t seed = 0;
		hash_combine(seed, get<0>(t));
		hash_combine(seed, get<1>(t));
		hash_combine(seed, get<2>(t));
		return seed;
	}
};

unordered_map<tuple<int, int, int>, long long, tuple_hash> memo;

static long long towerHeight(int n, int c, int lastSize, int lastColor, int fromCube, const vector<pair<int, int>>& cubes) {
	if (fromCube == n) return 0;

	// Pomijamy 
	long long height1 = 0;
	if (fromCube + 1 < n) {
		if (memo.find({ lastSize, lastColor, fromCube + 1 }) != memo.end())
			height1 = memo[{lastSize, lastColor, fromCube + 1}];
		else {
			height1 = towerHeight(n, c, lastSize, lastColor, fromCube + 1, cubes);
			memo[{lastSize, lastColor, fromCube + 1}] = height1;
		}
	}

	// Niepomijamy
	long long height2 = 0;
	if (lastSize < cubes[fromCube].first) {
		height2 = cubes[fromCube].first;
		if (lastColor > 0 && cubes[fromCube].second != lastColor)
			height2 -= c;
		if (fromCube + 1 < n) {
			long long h;
			if (memo.find({ cubes[fromCube].first, cubes[fromCube].second, fromCube + 1}) != memo.end())
				h = memo[{cubes[fromCube].first, cubes[fromCube].second, fromCube + 1}];
			else {
				h = towerHeight(n, c, cubes[fromCube].first, cubes[fromCube].second, fromCube + 1, cubes);
				memo[{cubes[fromCube].first, cubes[fromCube].second, fromCube + 1}] = h;
			}
			height2 += h;
		}
	}

	return (height1 > height2) ? height1 : height2;
}

int main()
{
	int n;
	int c;
	cin >> n >> c;

	vector<pair<int, int>> cubes;
	for (int i = 0; i < n; i++) {
		int a;
		int w;
		cin >> a >> w;
		cubes.push_back({ a,w });
	}

	auto start = chrono::steady_clock::now();

	long long height = towerHeight(n, c, 0, 0, 0, cubes);
	cout << height << endl;

	//auto end = chrono::steady_clock::now();
	//chrono::duration<double> elapsed_seconds = end - start;
	//cout << "Czas: " << elapsed_seconds.count() << endl;
}