#include <bits/stdc++.h>
#define pii pair<int, int>
using namespace std;
const int MAX = 400007;
int R[19][MAX];
int main() {
ios::sync_with_stdio(0); cin.tie(0);
int N, M;
cin >> N >> M;
vector<pii> T, ST;
for (int i = 1; i <= N; ++i) {
int a, b;
cin >> a >> b;
if (b < a) b += M;
T.emplace_back(a, b);
}
sort(T.begin(), T.end());
for (auto [a, b] : T) {
while (!ST.empty() && ST.back().first == a && ST.back().second <= b) ST.pop_back();
if (ST.empty() || ST.back().second < b) ST.emplace_back(a, b);
}
N = ST.size();
for (int i = 0; i < N; ++i) {
auto [a, b] = ST[i];
auto it = lower_bound(ST.begin(), ST.end(), pii{b, (int)2e9}) - ST.begin();
R[0][i] = it - 1;
assert(ST[R[0][i]].first <= b);
if (i) {
auto [c, d] = ST[i - 1];
assert(c < a && d < b);
}
}
for (int n = 1; n < 19; ++n) for (int i = 0; i < N; ++i) R[n][i] = R[n - 1][R[n - 1][i]];
int ans = 1e9;
for (int i = 0; i < N; ++i) {
int cur = i, goal = ST[cur].first + M, go = 1;
for (int n = 18; n >= 0; --n) {
auto [a, b] = ST[R[n][cur]];
if (b < goal) {
cur = R[n][cur];
go += 1 << n;
}
}
cur = R[0][cur];
if (goal <= ST[cur].second) ans = min(ans, go + 1);
}
cout << (ans == 1e9 ? -1 : ans);
return 0;
}
