package yukicoder;

import java.util.Scanner;
import static java.lang.Math.max;

public class No561 {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        int n = sc.nextInt(), d = sc.nextInt();
        int t, k, more, result = 0, resultK = -d;
        boolean tokyo = true;
        for(int i = 0; i < n; i++) {
            t = sc.nextInt();
            k = sc.nextInt();
            if( tokyo ) {
                more = max(t, k - d);
                if(t != k - d && more == k - d) tokyo = false;
                result += more;
            } else {
                more = max(t - d, k);
                if(t - d != k && more == t - d) tokyo = true;
                result += more;
            }
            resultK += k;
        }
        System.out.println(result < resultK ? resultK : result);
    }
}