#include "bits/stdc++.h"

#define REP(i,n) for(ll i=0;i<n;++i)
#define RREP(i,n) for(ll i=n-1;i>=0;--i)
#define FOR(i,m,n) for(ll i=m;i<n;++i)
#define RFOR(i,m,n) for(ll i=n-1;i>=m;--i)
#define ALL(v) (v).begin(),(v).end()
#define UNIQUE(v) v.erase(unique(ALL(v)),v.end());
#define DUMP(v) REP(aa, (v).size()) { cout << v[aa]; if (aa != v.size() - 1)cout << " "; else cout << endl; }
#define INF 1000000001ll
#define MOD 1000000007ll
#define EPS 1e-9

const int dx[8] = { 1,1,0,-1,-1,-1,0,1 };
const int dy[8] = { 0,1,1,1,0,-1,-1,-1 };


using namespace std;
typedef long long ll;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef vector<vi> vvi;
typedef vector<vl> vvl;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
ll max(ll a, int b) { return max(a, ll(b)); }
ll max(int a, ll b) { return max(ll(a), b); }
ll min(ll a, int b) { return min(a, ll(b)); }
ll min(int a, ll b) { return min(ll(a), b); }
///(´・ω・`)(´・ω・`)(´・ω・`)(´・ω・`)(´・ω・`)(´・ω・`)///
int n, d;
int dfs(vvi &dp, vi &k, vi &t, int i, int p) {
	if (i == n)return 0;
	if (dp[i][p] != -1)return dp[i][p];
	int ret = 0;
	if (p == 0) {//tokyo
		ret = max(ret, t[i] + dfs(dp, k, t, i + 1, 0));
		ret = max(ret, k[i] - d + dfs(dp, k, t, i + 1, 1));
	}
	else {
		ret = max(ret, k[i] + dfs(dp, k, t, i + 1, 1));
		ret = max(ret, t[i] - d + dfs(dp, k, t, i + 1, 0));
	}
	return dp[i][p] = ret;
}

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

	cin >> n >> d;
	vi t(n), k(n);
	REP(i, n)cin >> t[i] >> k[i];
	vvi dp(n, vi(2, -1));
	cout << dfs(dp, k, t, 0, 0) << endl;

}