from collections import deque
n,k = map(int,input().split())
LR = [list(map(int,input().split())) for i in range(n)]

par = [-1]*n

sLR = [[l,r,i] for i,(l,r) in enumerate(LR)]
sLR.sort(key=lambda x: x[1])
wait = deque([])
for l,r,ind in sLR:

    while wait and wait[0][1] <= l:
        bl,br,bind = wait.popleft()
        par[bind] = ind
    
    wait.append([l,r,ind])


doubling = [[-1]*n for i in range(20)]
doubling[0] = par[:]

for i in range(1,20):

    for j in range(n):
        pos = doubling[i-1][j]
        if pos == -1:
            continue
        pos = doubling[i-1][pos]
        doubling[i][j] = pos


ans = 10**10

for i in range(n):

    res = k-1
    now = i
    for j in range(20)[::-1]:
        if 1<<j <= res and now != -1:
            now = doubling[j][now]
            res -= 1<<j
    if now != -1:
        ans = min(ans,LR[now][1]-LR[i][0])


if ans == 10**10:
    ans = -1

print(ans)