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(); } }