#pragma warning(disable:4996)
#include <bits/stdc++.h>

using namespace std;

//macro
#define FOR(i,a,b) for(int i=(int)(a);i<(int)(b);i++)
#define REP(i,n) FOR(i,0,n)
#define each(i, ctn) for (auto &&i : (ctn))
#define all(a) (a).begin(),(a).end()
#define EPS (1e-8)
#define equals(a,b)(fabs((a)-(b))<EPS)

using ll = long long;
using ull = unsigned long long;
using vb = vector<bool>;
using vi = vector<int>;
using vll = vector<ll>;
using vs = vector<string>;
using vvb = vector<vb>;
using vvi = vector<vi>;
using vvll = vector<vll>;
//func
template<typename A, typename T>void assign2d(A&arr, ll H, ll W, T t) { arr.resize(H); REP(i, H) arr[i].assign(W, t); }
template<class T> ostream &operator<<(ostream &os, const vector<T> &v) { int n = v.size(); REP(i, n)os << v[i] << (i == n - 1 ? "" : " "); return os; }
template <class T = int> inline T IN() { T x; cin >> x; return x; }
template<class T> inline void OUT(const T &x) { cout << x << "\n"; }
struct pre_ { pre_() { cin.tie(nullptr); ios::sync_with_stdio(false); /*cout << fixed << setprecision(12);*/ } } pre__;


int dp[101][2];

int main() {
	int N, D;
	cin >> N >> D;
	vi T(N), K(N);
	REP(i, N) {
		cin >> T[i] >> K[i];
	}
	dp[0][0] = T[0];
	dp[0][1] = K[0] - D;

	FOR(i, 1, N) {

		dp[i][0] = max(dp[i - 1][0], dp[i - 1][1] - D) + T[i];
		dp[i][1] = max(dp[i - 1][1], dp[i - 1][0] - D) + K[i];
	}
	cout << max(dp[N - 1][0], dp[N - 1][1]) << endl;
	return 0;
}