import java.io.InputStream;
import java.io.PrintWriter;
import java.lang.reflect.Array;
import java.math.BigDecimal;
import java.util.ArrayDeque;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;
import java.util.PriorityQueue;
import java.util.Queue;
import java.util.Scanner;
import java.util.Set;
import java.util.Stack;
import java.util.TreeMap;
import java.util.TreeSet;
 
import static java.util.Comparator.*;
 
public class Main {
    public static void main(String[] args) {
        PrintWriter out = new PrintWriter(System.out);
        Solver solver = new Solver(System.in, out);
        solver.solve();
        out.close();
    }
}
class Solver {
	Scanner sc;
	PrintWriter out;
    public Solver(InputStream in, PrintWriter out) {
    	sc = new Scanner(in);
    	this.out = out;
    }
    // ==================================================================
    int N, D;
    int[] T, K;
    int calc(int now, boolean t) {
    	if(now == N)	return 0;
    	int wk1, wk2;
    	if(t) {
    		wk1 = calc(now + 1, true) + T[now];
    		wk2 = calc(now + 1, false) + K[now] - D;
    	} else {
    		wk1 = calc(now + 1, true) + T[now] - D;
    		wk2 = calc(now + 1, false) + K[now];
    	}
    	int ans = Math.max(wk1, wk2);
    	return ans;
    }
    public void solve() {
    	N = Integer.parseInt(sc.next());
    	D = Integer.parseInt(sc.next());
    	T = new int[N];
    	K = new int[N];
    	for (int i = 0; i < N; i++) {
			T[i] = Integer.parseInt(sc.next());;
			K[i] = Integer.parseInt(sc.next());;
		}
    	out.println(calc(0, true));
    }
}