#include<iostream>
#include<vector>
#include<algorithm>
#include<cctype>
#include<utility>
#include<string>
#include<cmath>
#include<cstring>
#include<queue>
#include<map>
#include<set>
#include<climits>
#include<bitset>
#include<stack>

#define REP(i, n) for(int i = 0;i < n;i++)
#define REPR(i, n) for(int i = n;i >= 0;i--)
#define FOR(i, m, n) for(int i = m;i < n;i++)
#define FORR(i, m, n) for(int i = m;i >= n;i--)
#define SORT(v, n) sort(v, v+n);
#define VSORT(v) sort(v.begin(), v.end());
#define llong long long
#define pb(a) push_back(a)
#define repi(itr, ds) for (auto itr = ds.begin(); itr != ds.end(); itr++)

using namespace std;

typedef long long int ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef tuple<ll, ll, ll, ll> lltpl;

int dy[8] = { 2,2,-2,-2,1,1,-1,-1 };
int dx[8] = { 1,-1,1,-1,2,-2,2,-2 };



/******************************************************************************************/


const int IINF = 1e9 + 7;
const ll LINF = 1e18 + 7;
const ll MOD = 998244353;



template<typename T>
vector<T> make_v(size_t a) { return vector<T>(a); }
template<typename T, typename... Ts>
auto make_v(size_t a, Ts... ts) {
	return vector<decltype(make_v<T>(ts...))>(a, make_v<T>(ts...));
}
template<typename T, typename V>
typename enable_if<is_class<T>::value == 0>::type
fill_v(T& t, const V& v) { t = v; }
template<typename T, typename V>
typename enable_if<is_class<T>::value != 0>::type
fill_v(T& t, const V& v) {
	for (auto& e : t) fill_v(e, v);
}

// vector
template <typename T>
istream& operator>>(istream & is, vector<T> & vec) { for (T& x : vec) is >> x; return is; }
// pair
template <typename T, typename U>ostream& operator<<(ostream & os, pair<T, U> & pair_var) { os << "(" << pair_var.first << ", " << pair_var.second << ")"; return os; }
// vector
template <typename T>ostream& operator<<(ostream & os, const vector<T> & vec) { os << "{";	for (int i = 0; i < vec.size(); i++) { os << vec[i] << (i + 1 == vec.size() ? "" : ", "); }os << "}"; return os; }
// map
template <typename T, typename U>ostream& operator<<(ostream & os, map<T, U> & map_var) { os << "{";	repi(itr, map_var) { os << *itr; itr++; if (itr != map_var.end()) os << ", "; itr--; }os << "}"; return os; }
// set
template <typename T>ostream& operator<<(ostream & os, set<T> & set_var) { os << "{"; repi(itr, set_var) { os << *itr; itr++; if (itr != set_var.end()) os << ", "; itr--; }os << "}"; return os; }



int dp[105][2];

int main() {

	cin.tie(nullptr);
	ios::sync_with_stdio(false);

	int N, D;
	cin >> N >> D;
	vector<int> T(N), K(N);
	for (int i = 0; i < N; i++)
	{
		cin >> T[i] >> K[i];
	}

	dp[0][1] = 0;
	for (int i = 0; i < N; i++)
	{
		for (int j = 0; j < 2; j++)
		{
			if (j == 0) {
				//tokyo
				dp[i + 1][0] = max(dp[i + 1][0], dp[i][0] + T[i]);

				//kyoto
				dp[i + 1][1] = max(dp[i + 1][1], dp[i][0] - D + K[i]);

			}
			else
			{
				//kyoto
				if (i == 0)continue;
				dp[i + 1][1] = max(dp[i + 1][1], dp[i][1] + K[i]);

				//tokyo
				dp[i + 1][0] = max(dp[i + 1][0], dp[i][1] - D + T[i]);
			}
		}
	}

	cout << max(dp[N][0], dp[N][1]) << endl;





}