#include <bits/stdc++.h>
#define F first
#define S second
using namespace std;
const int maxn = 2 * 1e5 + 7;
const int LOG = 20;
const long long MOD = (long long)(1e9) + 7;
const long long base = 311;
const int ALP_BET = 26;
typedef pair<int, int> ii;
typedef pair<int, long long> il;
typedef pair<long long, int> li;
typedef pair<long long, long long> ll;
struct RANGE{
int l, r;
};
bool cmp_RANGE(RANGE a, RANGE b){
return a.l < b.l;
}
int n, m;
RANGE a[maxn];
namespace SUB1{
struct ITEM{
int id, l, r;
};
bool cmp(ITEM a, ITEM b){
return a.l < b.l;
}
const int maxN = 20 + 7;
ii getid[maxN];
bool ok[maxN];
int nn; ITEM arr[2 * maxN];
int N; RANGE b[maxN];
void solve(){
nn = 0;
for(int i = 1; i <= n; ++i) if(a[i].l <= a[i].r){
++nn;
arr[nn] = {i, a[i].l, a[i].r};
} else{
++nn;
arr[nn] = {i, 1, a[i].r};
++nn;
arr[nn] = {i, a[i].l, m};
}
sort(arr + 1, arr + nn + 1, cmp);
for(int i = 1; i <= n; ++i) getid[i] = {-1, -1};
for(int i = 1; i <= nn; ++i){
int pos = arr[i].id;
if(getid[pos].F == -1)
getid[pos].F = i;
else{
getid[pos].S = getid[pos].F;
getid[pos].F = i;
}
}
// cerr << "--------------\n";
// cerr << nn << "\n";
// for(int i = 1; i <= nn; ++i){
// cerr << arr[i].id << " " << arr[i].l << " " << arr[i].r << "\n";
// }
// cerr << "--------------\n";
// for(int i = 1; i <= n; ++i){
// cerr << getid[i].F << " " << getid[i].S << "\n";
// }
int ans = n + 1;
int max_state = (1 << n);
for(int state = 1; state < max_state; ++state){
memset(ok, false, sizeof(ok));
for(int i = 0; i < n; ++i) if(state & (1 << i)){
ok[getid[i + 1].F] = true;
if(getid[i + 1].S != -1)
ok[getid[i + 1].S] = true;
}
N = 0;
for(int i = 1; i <= nn; ++i) if(ok[i]){
++N;
b[N] = {arr[i].l, arr[i].r};
}
bool check = true;
int Max = 0;
for(int i = 1; i <= N; ++i){
int l = b[i].l, r = b[i].r;
if(l - Max > 1){
check = false;
break;
}
Max = max(Max, r);
}
if(Max != m) check = false;
if(check == true){
ans = min(ans, __builtin_popcount(state));
}
}
if(ans == n + 1)
ans = -1;
cout << ans << "\n";
return;
}
}
int solve_st(RANGE a[]){
sort(a + 1, a + n + 1, cmp_RANGE);
int Maxx = 0; bool ok = true;
for(int i = 1; i <= n; ++i){
int l = a[i].l, r = a[i].r;
if(l - Maxx > 1){
ok = false;
break;
}
Maxx = max(Maxx, r);
}
if(Maxx != m) ok = false;
if(!ok){
return -1;
}
int ans = 0, Max = 0, MaxAll = 0;
for(int i = 1; i <= n; ++i){
int l = a[i].l, r = a[i].r;
if(l > Max + 1){
++ans;
Max = MaxAll;
}
MaxAll = max(MaxAll, r);
if(MaxAll == m){
++ans;
break;
}
}
return ans;
}
namespace SUB4{
void solve(){
int ans = solve_st(a);
cout << ans << "\n";
return;
}
}
int main()
{
ios::sync_with_stdio(false); cin.tie(0); cout.tie(0);
cin >> n >> m;
for(int i = 1; i <= n; ++i){
int l, r; cin >> l >> r;
r = (r - 1 + m) % m;
l = l + 1, r = r + 1;
a[i] = {l, r};
}
if(n <= 20){
SUB1::solve();
} else
SUB4::solve();
return 0;
}
