N, T = map(int, input().split())
spans = []
for _ in range(N):
spans.append(list(map(int, input().split())))
spans.sort()
r, nr, ans = 0, 0, 0
for cl, cr in spans:
if cr <= r: continue
if cl > nr + 1:ans = -1; break
if cl <= r + 1 and cr > nr: nr = cr
if cl > r: ans += 1; r = nr
if cl <= r + 1 and cr > nr: nr = cr
if ans != -1 and r != nr:
ans += 1
if nr < T: ans = -1
print(ans)