N,D = map(int,input().split())
pq = [list(map(int,input().split()))for _ in range(N)]
pq.sort(key=lambda x:(x[0],-x[1]))
l = [0]+sorted(list(map(lambda x:x[1]-x[0],pq)),reverse=True)
ans = 0
for i,x in enumerate(l[:-1]):
    ans = min(ans,x-pq[i][0])
print(ans)