import java.util.Scanner;

public class Main {

	@SuppressWarnings("resource")
	public static void main(String args[]) {
		Scanner scanner = new Scanner(System.in);
		int n=scanner.nextInt();
		int d=scanner.nextInt();
		t=new int[n];
		k=new int[n];
		for(int i=0;i<n;i++) {
			t[i]=scanner.nextInt();
			k[i]=scanner.nextInt();
		}
		dfs(0,0,true,d,n);
		System.out.println(max);
	}

	static int t[];
	static int k[];
	static int max=0;
	private static void dfs(int i, int c, boolean is_t, int d, int n) {
		if(i==n) {
			max=Math.max(max, c);
			return;
		}
		if(is_t) {
			dfs(i+1,c+t[i],true,d,n);
			dfs(i+1,c+k[i]-d,false,d,n);
		}else {
			dfs(i+1,c+t[i]-d,true,d,n);
			dfs(i+1,c+k[i],false,d,n);
		}
	}
}