import java.io.*;
import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner();
        int n = sc.nextInt();
        int d = sc.nextInt();
        int currentT = 0;
        int currentK = -d;
        while (n-- > 0) {
            int t = sc.nextInt();
            int k = sc.nextInt();
            int nextT = Math.max(currentT, currentK - d) + t;
            int nextK = Math.max(currentK, currentT - d) + k;
            currentT = nextT;
            currentK = nextK;
        }
        System.out.println(Math.max(currentT, currentK));
    }
}

class Scanner {
    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    StringTokenizer st = new StringTokenizer("");
    
    public Scanner() throws Exception {
        
    }
    
    public int nextInt() throws Exception {
        return Integer.parseInt(next());
    }
    
    public long nextLong() throws Exception {
        return Long.parseLong(next());
    }
    
    public double nextDouble() throws Exception {
        return Double.parseDouble(next());
    }
    
    public String next() throws Exception {
        while (!st.hasMoreTokens()) {
            st = new StringTokenizer(br.readLine());
        }
        return st.nextToken();
    }
}