#include "bits/stdc++.h"

using namespace std;

using ll = long long int;
using ld = long double;

using vi = vector<int>;
using vvi = vector<vi>;
using vvvi = vector<vvi>;
using vl = vector<ll>;
using vvl = vector<vl>;
using vvvl = vector<vvl>;
using vd = vector<double>;
using vvd = vector<vd>;
using vs = vector<string>;
using vb = vector<bool>;
using vvb = vector<vb>;

const int INF = (1 << 30) - 1;
const ll INF64 = ((ll)1 << 62) - 1;
const double PI = 3.1415926535897932384626433832795;

const int dx[] = { -1, 0, 1, 0 };
const int dy[] = { 0, 1, 0, -1 };

int gcd(int x, int y) { return y ? gcd(y, x % y) : x; }
ll gcd(ll x, ll y) { return y ? gcd(y, x % y) : x; }
int lcm(int x, int y) { return x / gcd(x, y) * y; }
ll lcm(ll x, ll y) { return x / gcd(x, y) * y; }

ll n, d;
vl t, k;

ll res = 0;
void takahasi(int i, bool tokyoPos, ll gold) {
	if (i >= n) {
		res = max(res, gold);
		return;
	}

	if (tokyoPos) {
		takahasi(i + 1, true, gold + t[i]);
		takahasi(i + 1, false, gold + k[i] - d);
	} else {
		takahasi(i + 1, true, gold + t[i] - d);
		takahasi(i + 1, false, gold + k[i]);
	}

	return;
}

int main() {
	ios::sync_with_stdio(false);
	cin.tie(0);

	cin >> n >> d;

	t.resize(n);
	k.resize(n);

	for (int i = 0; i < n; i++) {
		cin >> t[i] >> k[i];
	}

	takahasi(0, true, 0);

	cout << res << endl;
	return 0;
}