#include <iostream>
#include <vector>
#include <algorithm>
#include <limits>
using namespace std;
// Structure to represent an interval on the extended timeline.
struct Interval {
int start, end;
};
int main(){
ios::sync_with_stdio(false);
cin.tie(nullptr);
int N, M;
cin >> N >> M;
// We'll build our list of intervals in the extended timeline.
// Also, we collect candidate starting times (from the original intervals) that lie in [0, M).
vector<Interval> intervals;
vector<int> candidateT;
for (int i = 0; i < N; i++){
int s, e;
cin >> s >> e;
candidateT.push_back(s); // record the original start time
if(s <= e){
// Non-wrapping interval: add two copies to cover both "halves" of the doubled timeline.
intervals.push_back({s, e});
intervals.push_back({s + M, e + M});
} else {
// Wrapping interval: transform it into a single interval spanning from s to (e + M)
intervals.push_back({s, e + M});
}
}
// Sort the extended intervals by starting time.
sort(intervals.begin(), intervals.end(), [](const Interval &a, const Interval &b) {
return a.start < b.start;
});
// Remove duplicate candidate starting times.
sort(candidateT.begin(), candidateT.end());
candidateT.erase(unique(candidateT.begin(), candidateT.end()), candidateT.end());
int ans = numeric_limits<int>::max();
// For each candidate starting time T (must be in [0, M)),
// try to cover the interval [T, T + M] using a greedy algorithm.
for (int T : candidateT) {
if(T >= M) continue; // only consider candidate T within the first day
int count = 0;
int cur = T;
int target = T + M;
int j = 0; // pointer for scanning through intervals
// Greedy covering: extend coverage until we cover [T, T + M]
while(cur < target) {
int best = cur;
// While intervals start at or before our current time, choose the one that extends furthest.
while(j < intervals.size() && intervals[j].start <= cur) {
best = max(best, intervals[j].end);
j++;
}
// If we cannot extend our coverage, break out.
if(best == cur) {
count = -1;
break;
}
count++;
cur = best;
}
if(cur >= target && count != -1) {
ans = min(ans, count);
}
}
// If no candidate provided full coverage, output -1.
if(ans == numeric_limits<int>::max())
cout << -1 << "\n";
else
cout << ans << "\n";
return 0;
}
