def solve(n, k, LR):
    inf = 2 ** 63 - 1
    LR.sort()
    next = [[0] * n for _ in range(21)]
    
    d = []
    
    for i in range(n):
        d.append((LR[i][1], i))
        d.append((LR[i][0], i + n))
    d.sort(reverse=True)
    toind = -1
    
    for _, ind in d:
        if ind < n:
            next[0][ind] = toind
        else:
            ind -= n
            if toind < 0 or LR[toind][1] > LR[ind][1]:
                toind = ind
    for i in range(20):
        for j in range(n):
            if next[i][j] == -1:
                next[i + 1][j] = -1
            else:
                next[i + 1][j] = next[i][next[i][j]]
    ans = inf
    for i in range(n):
        now = i
        nowk = k - 1
        for j in range(20):
            if nowk % 2:
                now = next[j][now]
            nowk //= 2
        if now >= 0:
            x = LR[i][0]
            y = LR[now][1]
            ans = min(ans, y - x)
    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))