import sys
read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines

import numpy as np

data = np.fromstring(read(), np.int64, sep=' ')

N, K = data[:2]
AB = data[2:]
A = AB[::2]
B = AB[1::2]

def f(x):
    """count coins <= x"""
    return np.maximum(0, 1 + np.minimum(x, B) - A).sum()

left = 0
right = 10 ** 9
while left + 1 < right:
    x = (left + right) // 2
    if f(x) >= K:
        right = x
    else:
        left = x

print(right)