#include <stdio.h>
#include <assert.h>
#include <iostream>
#include <map>
#define MAXN 100000
using namespace std;

bool controlla_carica(int carica, map<int, bool> schema, int M) {
    //cout << "Controllo " << carica << endl;
    for (int i = 0; i < M; i++) {
        if (schema[i]) carica++;
        else carica--;
        if (carica <= 0) {
            return false;
        }
    }
    return true;
}
int ricarica(int N, int M, int A[], int B[]) {
    map<int, bool> schema;
    for (int i = 0; i < N; i++) {
        for (int j = A[i]; j <= B[i]; j++) {
            schema[j-1] = true;
        }
    }

    int first_carica, last_carica;
    int carica = 1;
    while (!controlla_carica(carica, schema, M)) {
        carica *= 2;
    }
    first_carica = carica / 2;
    last_carica = carica - 1;
    while (first_carica < last_carica) {
        int middle = first_carica + (last_carica - first_carica) / 2 + 1;
        if (!controlla_carica(middle, schema, M)) {
            first_carica = middle;
        }
        else {
            last_carica = middle - 1;
        }
    }
    first_carica++;
    //cout << first_carica << endl;
    return first_carica;

}



int A[MAXN], B[MAXN];

int main() {
    FILE *fr, *fw;
    int N, M, i;

    fr = fopen("input.txt", "r");
    fw = fopen("output.txt", "w");
    assert(2 == fscanf(fr, "%d %d", &N, &M));
    for(i=0; i<N; i++)
        assert(2 == fscanf(fr, "%d %d", &A[i], &B[i]));

    fprintf(fw, "%d\n", ricarica(N, M, A, B));
    fclose(fr);
    fclose(fw);
    return 0;
}