#include <bits/stdc++.h>
#define ll long long
#define task "FIRE"
using namespace std;
const int N = 1e6 + 16;
const ll INF = 1e18;
int n, m;
ll s[N], e[N];
namespace sub3 {
ll cnt, result = INF;
pair <ll, ll> a[N];
vector < pair <ll, ll> > checkLines;
bool cmp(pair <ll, ll> a, pair <ll, ll> b) {
if (a.first == b.first) {
return a.second > b.second;
}
return a.first < b.first;
}
ll query(pair <ll, ll> target) {
ll curP = 1, p = target.first, cnt = 0;
while (p <= target.second) {
ll r = -1;
while (curP <= n && a[curP].first <= p) {
r = max(a[curP].second, r);
curP++;
}
if (r < p) {
return -1;
}
p = r + 1;
cnt++;
}
return cnt;
}
void solve() {
checkLines.push_back({0, m - 1});
for (int i = 1; i <= n; i++) {
if (s[i] > e[i]) {
checkLines.push_back({e[i], e[i] + m - 1});
e[i] += m;
}
a[i] = {s[i], e[i] - 1};
}
sort(a + 1, a + 1 + n, cmp);
for (auto x : checkLines) {
ll ans = query(x);
if (ans == -1) {
continue;
}
result = min(result, ans);
}
if (result == INF) {
result = -1;
}
cout << result;
}
}
namespace sub6 {
ll cnt, result = INF;
pair <ll, ll> a[N];
vector < pair <ll, ll> > checkLines;
ll maxx;
void compress() {
vector <ll> c;
for (int i = 1; i <= n; i++) {
c.push_back(a[i].first);
c.push_back(a[i].second);
}
for (auto x : checkLines) {
c.push_back(x.first);
c.push_back(x.second);
}
sort(c.begin(), c.end());
c.erase(unique(c.begin(), c.end()), c.end());
for (int i = 1; i <= n; i++) {
a[i].first = lower_bound(c.begin(), c.end(), a[i].first) - c.begin() + 1;
a[i].second = lower_bound(c.begin(), c.end(), a[i].second) - c.begin() + 1;
maxx = max(maxx, a[i].first);
maxx = max(maxx, a[i].second);
}
for (auto &x : checkLines) {
x.first = lower_bound(c.begin(), c.end(), x.first) - c.begin() + 1;
x.second = lower_bound(c.begin(), c.end(), x.second) - c.begin() + 1;
maxx = max(maxx, x.first);
maxx = max(maxx, x.second);
}
}
ll maxR[N], up[N][26];
void solve() {
checkLines.push_back({0, m - 1});
for (int i = 1; i <= n; i++) {
if (s[i] > e[i]) {
checkLines.push_back({e[i], e[i] + m - 1});
e[i] += m;
}
a[i] = {s[i], e[i] - 1};
}
compress();
for (int i = 1; i <= n; i++) {
maxR[a[i].first] = a[i].second;
}
for (int i = 1; i <= maxx; i++) {
maxR[i] = max(maxR[i - 1], maxR[i]);
}
up[maxx][0] = maxx;
for (int i = maxx - 1; i >= 1; i--) {
if (maxR[i] != maxR[i + 1]) {
up[i][0] = i;
}
else {
up[i][0] = up[i + 1][0];
}
}
for (int j = 1; j <= 20; j++) {
for (int i = 1; i <= maxx; i++) {
if (1 <= up[i][j - 1] && up[i][j - 1] <= maxx) {
up[i][j] = up[up[i][j - 1] + 1][j - 1];
}
}
}
for (auto x : checkLines) {
int u = x.first, v = x.second, ans = 0;
while (u <= v) {
int k = -1;
for (int i = 20; i >= 0; i--) {
if (up[u][i] == 0) {
continue;
}
if (up[u][i] < v) {
k = i;
break;
}
}
if (k == -1) {
break;
}
u = up[u][k] + 1;
ans += (1 << k);
}
ans++;
result = min(result, 1LL * ans);
}
if (result > n) {
result = -1;
}
cout << result;
}
}
int main() {
ios_base :: sync_with_stdio(false);
cin.tie(0);
cout.tie(0);
if (fopen(task".inp", "r")) {
freopen(task".inp", "r", stdin);
freopen(task".out", "w", stdout);
}
cin >> n >> m;
for (int i = 1; i <= n; i++) {
cin >> s[i] >> e[i];
}
if (n <= 5000) {
sub3 :: solve();
}
else {
sub6 :: solve();
}
return 0;
}
