import sys

sys.setrecursionlimit(200005)
# sys.set_int_max_str_digits(200005)
int1 = lambda x: int(x)-1
pDB = lambda *x: print(*x, end="\n", file=sys.stderr)
p2D = lambda x: print(*x, sep="\n", end="\n\n", file=sys.stderr)
def II(): return int(sys.stdin.readline())
def LI(): return list(map(int, sys.stdin.readline().split()))
def LLI(rows_number): return [LI() for _ in range(rows_number)]
def LI1(): return list(map(int1, sys.stdin.readline().split()))
def LLI1(rows_number): return [LI1() for _ in range(rows_number)]
def SI(): return sys.stdin.readline().rstrip()

# dij = [(0, 1), (-1, 0), (0, -1), (1, 0)]
dij = [(0, 1), (-1, 0), (0, -1), (1, 0), (1, 1), (1, -1), (-1, 1), (-1, -1)]
# inf = -1-(-1 << 31)
inf = -1-(-1 << 62)

# md = 10**9+7
md = 998244353

from collections import deque

n,k=LI()
q=deque()
p=-1
lr=LLI(n)
lr.sort(key=lambda x:x[0])
to=[[] for _ in range(n)]
for u,(l,r) in enumerate(lr):
    while q and q[0][-1]<=l:p,_=q.popleft()
    if p!=-1:to[p].append(u)
    while q and q[-1][-1]>=r:q.pop()
    q.append((u,r))
# print(lr)
# print(to)

fin=[0]*n
ans=inf
uu=[]

def dfs(u):
    global ans
    fin[u]=1
    uu.append(u)
    r = lr[u][1]
    if len(uu)>=k:
        v=uu[-k]
        l=lr[v][0]
        ans=min(ans,r-l)
    for v in to[u]:
        dfs(v)
    uu.pop()

for u in range(n):
    if fin[u]:continue
    dfs(u)

print(-1 if ans==inf else ans)