import sys
input = sys.stdin.readline

N, K = map(int, input().split())
LR = []
for i in range(N):
    L, R = map(int, input().split())
    LR.append((L, R))

D = []
for i in range(N):
    D.append((LR[i][0], i+N))
    D.append((LR[i][1], i))
    
D.sort(reverse=True)
M = 20
dp = [[-1] * N for i in range(M)]
nex = -1
for _, d in D:
    if d < N:
        dp[0][d] = nex
    else:
        d -= N
        if nex == -1 or LR[d][1] <= LR[nex][1]:
            nex = d
            
for i in range(1, M):
    for j in range(N):
        if dp[i-1][j] != -1:
            dp[i][j] = dp[i-1][dp[i-1][j]]
        
inf = 10 ** 18
ans = inf
for i in range(N):
    now = i
    for j in range(M):
        if ((K - 1) >> j) & 1:
            now = dp[j][now]
            if now == -1:
                break
            
    if now != -1:
        ans = min(ans, LR[now][1] - LR[i][0])
        
print(ans) if ans != inf else print(-1)