#include "bits/stdc++.h"

using namespace std;
using ll = long long;
const double pi = acos(-1);
#define FOR(i,a,b) for (ll i=(a),__last_##i=(b);i<__last_##i;i++)
#define RFOR(i,a,b) for (ll i=(b)-1,__last_##i=(a);i>=__last_##i;i--)
#define REP(i,n) FOR(i,0,n)
#define RREP(i,n) RFOR(i,0,n)
#define __GET_MACRO3(_1, _2, _3, NAME, ...) NAME
#define rep(...) __GET_MACRO3(__VA_ARGS__, FOR, REP)(__VA_ARGS__)
#define rrep(...) __GET_MACRO3(__VA_ARGS__, RFOR, RREP)(__VA_ARGS__)
template<typename T> ostream& operator<<(ostream& os, const vector<T>& v) {
	REP(i, v.size()) { if (i)os << " "; os << v[i]; }return os;
}
template<typename T> ostream& operator<<(ostream& os, const vector<vector<T>>& v) {
	REP(i, v.size()) { if (i)os << endl; os << v[i]; }return os;
}

const ll INF = 1LL << 60;
ll MOD = 1000000007;
ll _MOD = 1000000009;
double EPS = 1e-10;

#define int long long

inline void my_io() {

	std::ios::sync_with_stdio(false);

	std::cin.tie(0);

	cout << fixed << setprecision(16);

	//cout << setprecision(10) << scientific << ans << endl;
}

signed main() {
	ll n, d, t, k;

	cin >> n >> d;

	vector<pair<ll, ll>> v;

	REP(i, n) {
		cin >> t >> k;
		v.push_back(make_pair(t, k));
	}

	vector<vector<ll>> dp(n, vector<ll>(2, 0));
	dp[0][0] = v[0].first;
	dp[0][1] = v[0].second - d;

	FOR(i, 1, n) {
		dp[i][0] = v[i].first + max(dp[i - 1][0], dp[i - 1][1] - d);
		dp[i][1] = v[i].second + max(dp[i - 1][0] - d, dp[i - 1][1]);
	}

	cout << max(dp[n - 1][0], dp[n - 1][1]) << endl;
}