#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define MOD 998244353
const int N = 5005;
const int lg = 13;
ll n, m, s[N], e[N];
int bst[N];
int sp[N][lg];
void solve(){
cin >> n >> m;
for (int i = 0; i < n; i++){
cin >> s[i] >> e[i];
}
for (int i = 0; i < n; i++) {
if (s[i] > e[i]){
e[i] += m;
}
}
for (int i = 0; i < n; i++) {
s[i+n] = s[i] + m;
e[i+n] = e[i] + m;
}
n *= 2;
int cnt = 0;
vector<pair<ll, ll>> ranges;
for (int i = 0; i < n; i++)
ranges.push_back({s[i], -e[i]});
sort(ranges.begin(), ranges.end());
ll last = -1;
for (auto j : ranges) {
ll start = j.first;
ll end = -j.second;
if (end <= last) {
continue;
}
last = end;
s[cnt] = start;
e[cnt] = end;
cnt++;
}
n = cnt;
ranges.clear();
for (int i = 0; i < n; i++)
ranges.push_back({s[i], i});
sort(ranges.begin(), ranges.end());
int i = 0;
for (auto j : ranges) {
while (i < n) {
if (s[ranges[i].second] > e[j.second]){
break;
}
i++;
}
bst[j.second] = ranges[i - 1].second;
}
for (int i = 0; i < n; i++){
sp[i][0] = bst[i];
}
for (int k = 1; k < lg; k++)
for (int i = 0; i < n; i++)
sp[i][k] = sp[sp[i][k-1]][k-1];
int ans = -1;
for (int i = 0; i < n; i++) {
ll st = s[i];
int mn = 0;
int rt = i;
for (int i = lg-1; i >= 0; i--) {
if (e[sp[rt][i]] < (st + m)) {
mn += (1<<i);
rt = sp[rt][i];
}
}
mn++;
rt = sp[rt][0];
if (e[rt] < (st + m)) {
continue;
}
if (ans == -1 or mn + 1 < ans) ans = mn + 1;
}
cout << ans << endl;
}
int main() {
ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
int tests = 1;
// cin >> tests;
for(int i = 1; i <= tests; i++){
solve();
}
}
