#coins donation
from collections import deque
N, K = map(int, input().split())
d = []
for i in range(N):
    a, b = map(int, input().split())
    d.append((a, b))
#二部探索


def cnt(X):
    res = 0
    for a, b in d:
        if X >= a:
            res += min(X-a+1, b-a+1)
    return res


l = 0
r = 10**10
#l :::: cnt(X)<K
#r :::: cnt(X)>=K

while r-l > 1:
    M = (r+l)//2
    if cnt(M) >= K:
        r = M
    else:
        l = M
print(r)