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

public class Main {
	public static void main (String[] args) throws java.lang.Exception
	{
		// your code goes here
		// 入力
		FastScanner sc = new FastScanner(System.in);
		int N = sc.nextInt();
		int D = sc.nextInt();
		int[] T = new int[N];
		int[] K = new int[N];
		int[][] income = new int[2][N+1];
		for(int i = 0; i < N; i++){
			T[i] = sc.nextInt();
			K[i] = sc.nextInt();
		}
		
		// 収入表の作成
		for(int i = 0; i < N; i++){
			if(i == 0){
				income[0][i+1] = T[i];
				income[1][i+1] = max(0,K[i] - D);
			}else{
				income[0][i+1] = max(income[0][i] + T[i], income[1][i] + T[i] - D);
				income[1][i+1] = max(income[0][i] + K[i] - D, income[1][i] + K[i]);
			}
		}
		
		// 出力
		System.out.println(max(income[0][N], income[1][N]));
	}
	
	// 2数の最大を取得する関数
	public static int max(int a, int b){
		if(a > b){
			return a;
		}
		return b;
	}


	// 高速スキャナー
    static class FastScanner {
        private BufferedReader reader = null;
        private StringTokenizer tokenizer = null;

        public FastScanner(InputStream in) {
            reader = new BufferedReader(new InputStreamReader(in));
            tokenizer = null;
        }

        public String next() {
            if (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        }

        public String nextLine() {
            if (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    return reader.readLine();
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }

            return tokenizer.nextToken("\n");
        }

        public long nextLong() {
            return Long.parseLong(next());
        }

        public int nextInt() {
            return Integer.parseInt(next());
        }

        public double nextDouble() {
             return Double.parseDouble(next());
         }

        public int[] nextIntArray(int n) {
            int[] a = new int[n];
            for (int i = 0; i < n; i++)
                a[i] = nextInt();
            return a;
        }

        public long[] nextLongArray(int n) {
            long[] a = new long[n];
            for (int i = 0; i < n; i++)
                a[i] = nextLong();
            return a;
        } 
    }
}