#include<stdio.h>
#include<stdlib.h>
#include<stdint.h>
#include<inttypes.h>

typedef int32_t i32;
typedef int64_t i64;

#define MIN(a,b) ((a) < (b) ? (a) : (b))

void run (void) {
  i32 n, k;
  scanf ("%" SCNi32 "%" SCNi32, &n, &k);
  i32 *a = (i32 *) calloc (2 * n, sizeof (i32));
  i32 *b = a + n;
  for (i32 i = 0; i < n; ++i) {
    scanf ("%" SCNi32 "%" SCNi32, a + i, b + i);
  }
  i32 l = 0;
  i32 r = 1000000000;
  while (r - l > 1) {
    i32 m = (l + r) / 2;
    i64 cnt = 0;
    for (i32 i = 0; i < n; ++i) {
      if (m < a[i]) continue;
      cnt += MIN(m - a[i] + 1, b[i] - a[i] + 1);
    }
    if (cnt >= k) {
      r = m;
    } else {
      l = m;
    }
  }
  printf ("%" PRIi32 "\n", r);
}

int main (void) {
  run();
  return 0;
}