#include <bits/stdc++.h>
using namespace std;
#define int long long
signed main() {
int n, m; cin >> n >> m;
vector<pair<int, int>> shifts;
int max_dist = 0;
int start_x = 0;
for (int i = 0; i < n; i++) {
int x, y; cin >> x >> y;
if (x > y) y += m;
if (y-x > max_dist) {max_dist = y-x;start_x = x;}
shifts.push_back({x, y});
}
for (int i=0; i < n; i++) {
shifts[i] = {(shifts[i].first-start_x+m)%m, (shifts[i].second-start_x+m)%m};
if (shifts[i].first > shifts[i].second) shifts[i]={shifts[i].first, shifts[i].second+m};
}
sort(shifts.begin(), shifts.end());
int time = 0;
int furthest_time = 0;
int c = 0;
for (int i=0; i < n; i++) {
if (shifts[i].first <= time) furthest_time = max(furthest_time, shifts[i].second);
else {
c++;
time = furthest_time;
if (time >= m) {cout << c; return 0;}
if (shifts[i].first > time) {cout << -1; return 0;}
furthest_time = shifts[i].second;
}
}
c++;
time = furthest_time;
if (time >= m) {cout << c; return 0;}
else cout << -1;
}
