#include<iostream>
#include<iomanip>
#include<math.h>
#include<vector>
#include<algorithm>
#include<set>
#include<map>
#include<queue>
#include<stack>
#include<string>
#include<complex>
#include<random>
#include<time.h>

#define INF 1000000000ll
#define MOD 1000000007ll
#define EPS 1e-8

#define REP(i, m) for(long long i = 0; i < m; ++i)
#define FOR(i, n, m) for(long long i = n; i < m; ++i)
#define ALL(v) v.begin(), v.end()
#define pb push_back

using namespace std;
typedef long long ll;
typedef pair<ll, ll> P;
typedef long double ld;


int main() {
	ios::sync_with_stdio(false);
	ll n,d;
	cin>>n>>d;
	vector<P> tk(n);
	REP(i,n) cin>>tk[i].first>>tk[i].second;
	vector<P> dp(n);
	REP(i,n) {
		if(i==0) {
			dp[i].first=tk[i].first;
			dp[i].second=tk[i].second-d;
		} else {
			dp[i].first=tk[i].first+max(dp[i-1].first,dp[i-1].second-d);
			dp[i].second=tk[i].second+max(dp[i-1].second,dp[i-1].first-d);
		}
	}
	cout<<max(dp[n-1].first,dp[n-1].second)<<endl;
}