from collections import deque

N, K = map(int, input().split())
LR = sorted([list(map(int, input().split())) for _ in range(N)], key=lambda x:x[1])

nex = [N]*N
que = deque()
for i, (L, R) in enumerate(LR):
    while que and que[0][0] <= L:
        n, idx = que.popleft()
        nex[idx] = i
    que.append((R, i))

nex = [nex[:]]
for i in range(29):
    A = []
    for j in range(N):
        if nex[i][j] < N:
            A.append(nex[i][nex[i][j]])
        else:
            A.append(N)
    nex.append(A)

ans = 10**18
K -= 1
for i in range(N):
    goal = i
    for j in range(30):
        if 1<<j & K:
            goal = nex[j][goal]
            if goal == N:
                break
    if goal < N:
        L1, R1 = LR[i]
        L2, R2 = LR[goal]
        ans = min(ans, R2-L1)

print(ans if ans != 10**18 else -1)