766 B | 766 chars
import sys
from math import sqrt, pi, sin, factorial, ceil, floor
from decimal import Decimal
BLANK = " "
# inp = input
inp = lambda : sys.stdin.readline()[:-1].strip()
mii = lambda x = BLANK : [*map(int,inp().split(x))]
mfi = lambda x = BLANK : [*map(float,inp().split(x))]
ii = lambda : int(inp())
fi = lambda : float(inp())
p = print
def solve():
n, k = mii()
s = 0
e = 11111111111111111111111111
ans = e
l = [mii() for _ in range(n)]
while s <= e:
X = (s + e) // 2
cnt = 0
for a, b in l:
if a + X >= b:
cnt += 1
if cnt >= k:
ans = min(ans, X)
e = X - 1
else:
s = X + 1
p(ans)
if __name__ == "__main__":
tc = 1
for t in range(1, tc+1):
ret = solve()