#嘘解法
def solve(n, k, LR):
    LR.sort(key = lambda x: x[1])
    inf = 2 ** 63 - 1
    ans = inf
    D = []
    now = -1
    for l, r in LR:
        if l > now:
            now = r
            D.append((l, r))
    for i in range(len(D)):
        if i + k - 1 < len(D):
            ans = min(ans, D[i + k - 1][1] - D[i][0])
    return ans if ans < inf else -1
        

def solve_gu(n, k, LR):
    import itertools
    ans = inf
    for v in itertools.combinations(LR, k):
        v = list(v)
        v.sort(key = lambda x: x[1])
        
        now = -1
        f = True
        for l, r in v:
            if l < now:
                f = False
            now = r
        if f:
            ans = min(ans, v[-1][-1] - v[0][0])
    return ans if ans < inf else -1
import sys, time, random
from collections import deque, Counter, defaultdict
input = lambda: sys.stdin.readline().rstrip()
ii = lambda: int(input())
mi = lambda: map(int, input().split())
li = lambda: list(mi())
inf = 2 ** 63 - 1
mod = 998244353

n, k = mi()
LR = [li() for _ in range(n)]

print(solve(n, k, LR))