import java.math.BigDecimal;
import java.util.Arrays;
import java.util.LinkedList;
import java.util.Scanner;
import java.util.TreeSet;

public class Main {
	
	public static long MOD = 1000000007;
	
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		
		final int N = sc.nextInt();
		final long D = sc.nextLong();
		
		long[][] DP = new long[N + 1][2];
		
		DP[0][0] = 0;
		DP[0][1] = -D;
		// 資金がショートしないから初期化が適当でも大丈夫
		
		for(int i = 0; i < N; i++){
			final int T = sc.nextInt();
			final int K = sc.nextInt();
			
			DP[i + 1][0] = Math.max(DP[i + 1][0], DP[i][0] - 0 + T);
			DP[i + 1][0] = Math.max(DP[i + 1][0], DP[i][1] - D + T);
			
			DP[i + 1][1] = Math.max(DP[i + 1][1], DP[i][1] - 0 + K);
			DP[i + 1][1] = Math.max(DP[i + 1][1], DP[i][0] - D + K);
		}
		//System.out.println(Arrays.deepToString(DP));
		
		System.out.println(Math.max(DP[N][0], DP[N][1]));
	}
}