#include <bits/stdc++.h>
using namespace std;
using ll = long long;
const int INF = 1e9;
const ll LINF = 4e18;
const int MOD = 1'000'000'007; // not needed for this task
struct Seg { ll l, r; }; // [l, r] (inclusive)
int main() {
ios::sync_with_stdio(false);
cin.tie(nullptr);
int N;
ll M;
if (!(cin >> N >> M)) return 0;
vector<Seg> segs; segs.reserve(2 * N);
for (int i = 0; i < N; ++i) {
ll s, e; cin >> s >> e;
if (s <= e) {
segs.push_back({s, e});
segs.push_back({s + M, e + M}); // duplicate for wrap
} else {
segs.push_back({s, e + M}); // overnight
}
}
sort(segs.begin(), segs.end(),
[](const Seg& a, const Seg& b) { return a.l < b.l; });
const int SZ = segs.size();
vector<int> prefBest(SZ); // index of max‑end so far
prefBest[0] = 0;
for (int i = 1; i < SZ; ++i) {
prefBest[i] =
(segs[i].r > segs[prefBest[i - 1]].r) ? i : prefBest[i - 1];
}
/* next[i] : greedy successor */
vector<int> nxt(SZ, -1);
for (int i = 0; i < SZ; ++i) {
ll reach = segs[i].r;
int pos = int(upper_bound(segs.begin(), segs.end(), reach,
[](ll x, const Seg& s) { return x < s.l; })
- segs.begin()) - 1;
if (pos >= 0) {
int cand = prefBest[pos];
if (segs[cand].r > segs[i].r) nxt[i] = cand;
}
}
/* binary lifting */
const int LOG = 18; // 2^18 > 4*10^5
vector<array<int, LOG + 1>> up(SZ);
vector<array<ll , LOG + 1>> far(SZ);
for (int i = 0; i < SZ; ++i) {
up[i][0] = nxt[i];
far[i][0] = segs[i].r;
}
for (int k = 1; k <= LOG; ++k) {
for (int i = 0; i < SZ; ++i) {
int mid = up[i][k - 1];
if (mid == -1) {
up[i][k] = -1;
far[i][k] = far[i][k - 1];
} else {
up[i][k] = up[mid][k - 1];
far[i][k] = max(far[i][k - 1], far[mid][k - 1]);
}
}
}
/* try every interval whose left end is within the first day */
int bestAns = INF;
for (int i = 0; i < SZ && segs[i].l < M; ++i) {
ll goal = segs[i].l + M;
if (segs[i].r >= goal) { // covers whole day alone
bestAns = 1;
continue;
}
int cur = i, used = 1;
for (int k = LOG; k >= 0; --k) {
if (up[cur][k] != -1 && far[cur][k] < goal) {
used += 1 << k;
cur = up[cur][k];
}
}
int nx = nxt[cur];
if (nx != -1 && segs[nx].r >= goal) {
bestAns = min(bestAns, used + 1);
}
}
cout << (bestAns == INF ? -1 : bestAns) << '\n';
return 0;
}
