#include <bits/stdc++.h>
using namespace std;
typedef long long un;
typedef vector<un> vuc;
typedef pair<un, un> pun;
#define REP(i, a, b) for (un i = (un)a; i < (un)b; i++)
#define FEAC(i, a) for (auto&& i : a)
#define vec vector
#define ALL(x) (x).begin(), (x).end()
#define DEB(x) cerr << #x << " = " << (x) << endl;
int main(){
un N, M;
cin >> N >> M;
vec<pun> vstup;
map<un, un> mapa;
REP(i, 0, N){
un a, b;
cin >> a >> b;
vstup.push_back({a, b});
mapa[a] = 0;
mapa[b] = 0;
}
vuc imapa;
un counter = 0;
FEAC(kv, mapa){
un key;
tie(key, ignore) = kv;
mapa[key] = counter;
counter++;
imapa.push_back(key);
}
sort(ALL(vstup));
un K = imapa.size();
vuc delky(K, 0);
FEAC(kd, vstup){
un key, destination;
tie(key, destination) = kd;
delky[mapa[key]] = (M+destination-key) % M;
}
un curr = 0;
REP(ign, 0, 2){
REP(i, 0, K){
curr -= (M+imapa[i] - imapa[(K+i-1)%K]) % M;
curr = max(curr, delky[i]);
delky[i] = curr;
}
}
vuc visited(K, 0);
un ptr = 0;
un time = 1;
while(not visited[ptr]){
visited[ptr] = time;
if (delky[ptr] == 0) {
cout << -1 << endl;
return 0;
}
ptr = mapa[(imapa[ptr] + delky[ptr]) % M];
time++;
}
un ret = 2;
un goal = ptr;
ptr = mapa[(imapa[ptr] + delky[ptr]) % M];
un next_ptr = mapa[(imapa[ptr] + delky[ptr]) % M];
while(((M + imapa[goal] - imapa[ptr]) % M) + ((M + imapa[next_ptr] - imapa[goal]) % M) >= M){
ptr = next_ptr;
next_ptr = mapa[(imapa[ptr] + delky[ptr]) % M];
ret++;
}
cout << ret << endl;
}
