import java.io.; import java.util.;

public class Solution {

static class Pair{
    int a,b;
    Pair(int a, int b){
        this.a = a;
        this.b = b;
    }
}
static int circleDist(int a, int b,int c){

    int d = Math.abs(a-b);
    return Math.min(d, c-d);
}

static int pairDist(Pair p1, Pair p2, int c){

    int d1 = circleDist(p1.a, p2.a, c); 
    int d2 = circleDist(p1.a, p2.b, c);
    int d3 = circleDist(p1.b, p2.a, c);
    int d4 = circleDist(p1.b, p2.b, c);
    return Math.min(Math.min(d1, d2), Math.min(d3, d4));

}

public static void main(String[] args) throws IOException {
    BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(System.in));

    String[] firstMultipleInput = bufferedReader.readLine().replaceAll("\\s+$", "").split(" ");

    int n = Integer.parseInt(firstMultipleInput[0]);

    int c = Integer.parseInt(firstMultipleInput[1]);

    List<Pair> pair = new ArrayList<>();

     for(int i=0; i<n; i++){
        String[] line = bufferedReader.readLine().split(" "); 
        int a = Integer.parseInt(line[0]);
        int b = Integer.parseInt(line[1]);
        pair.add(new Pair(a, b));
        //points.add(new Pair(b, i));
     }          

    int maxDist = 0;
    for(int i=0; i<n; i++){
        for(int j=i+1; j<n; j++){
            int d = pairDist(pair.get(i), pair.get(j), c);
            maxDist = Math.max(maxDist, d);
        }

    }
    System.out.println(maxDist);
    bufferedReader.close();

 }

}