#include "bits/stdc++.h"
using namespace std;
#ifdef _DEBUG
#include "dump.hpp"
#else
#define dump(...)
#endif

#define int long long
#define rep(i,a,b) for(int i=(a);i<(b);i++)
#define rrep(i,a,b) for(int i=(b)-1;i>=(a);i--)
#define all(c) begin(c),end(c)
const int INF = sizeof(int) == sizeof(long long) ? 0x3f3f3f3f3f3f3f3fLL : 0x3f3f3f3f;
const int MOD = 1'000'000'007;
template<class T> bool chmax(T &a, const T &b) { if (a < b) { a = b; return true; } return false; }
template<class T> bool chmin(T &a, const T &b) { if (b < a) { a = b; return true; } return false; }

signed main() {
	cin.tie(0);
	ios::sync_with_stdio(false);
	int N, D; cin >> N >> D;
	vector<int> T(N), K(N); rep(i, 0, N) {
		cin >> T[i] >> K[i];
	}
	vector<vector<int>> dp(N + 1, vector<int>(2));
	dp[0][1] = INF;
	rep(i, 0, N) {
		chmax(dp[i + 1][0], dp[i][0] + T[i]);
		chmax(dp[i + 1][1], dp[i][0] + K[i] - D);
		if (dp[i][1] == INF)continue;
		chmax(dp[i + 1][0], dp[i][1] + T[i] - D);
		chmax(dp[i + 1][1], dp[i][1] + K[i]);
	}
	cout << max(dp[N][0], dp[N][1]) << endl;
	return 0;
}