#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define fti(i, x, y) for (int i = x; i <= y; ++i)
#define F first
#define S second
int n, m, res = 0, lg;
pair <int, int> t[200005];
int s[200005], e[200005], up[200005][20];
int tree[800005];
void zip()
{
vector <int> z;
z.push_back(m);
fti(i, 1, n)
z.push_back(t[i].F), z.push_back(t[i].S);
z.push_back(1e9 + 7);
sort(z.begin(), z.end());
z.erase(unique(z.begin(), z.end()), z.end());
m = lower_bound(z.begin(), z.end(), m) - z.begin();
fti(i, 1, n)
{
t[i].F = lower_bound(z.begin(), z.end(), t[i].F) - z.begin();
t[i].S = lower_bound(z.begin(), z.end(), t[i].S) - z.begin();
}
}
//
int get(int x, int y)
{
return (t[x].S < t[y].S ? y : x);
}
void build(int nd = 1, int l = 1, int r = n)
{
if (l == r)
{
tree[nd] = l; return;
}
int c = (l + r) >> 1, p1 = nd << 1, p2 = p1 | 1;
build(p1, l, c); build(p2, c + 1, r);
tree[nd] = get(tree[p1], tree[p2]);
}
int query(int L, int R, int nd = 1, int l = 1, int r = n)
{
if (r < L || R < l) return 0;
if (L <= l && r <= R) return tree[nd];
int c = (l + r) >> 1, p1 = nd << 1, p2 = p1 | 1;
return get(query(L, R, p1, l, c), query(L, R, p2, c + 1, r));
}
bool ok[51];
void suf1()
{
int msk = (1 << n) - 1;
fti(mask, 1, msk)
{
bool chek = 1;
fti(i, 0, m) ok[i] = 0;
fti(i, 1, n)
if ((mask >> (i - 1)) & 1)
{
fti(u, t[i].F, min(t[i].S, m)) ok[u] = 1;
}
fti(i, 0, m)
if (!ok[i])
{
chek = 0; break;
}
if (chek) res = min(res, __builtin_popcount(mask));
}
cout << (res > n ? -1 : res);
}
void sus2()
{
build();
fti(i, 1, n)
{
pair <int, int> nxt = {t[i].S + 1, 0};
int pos = lower_bound(t + 1, t + n + 1, nxt) - t - 1;
up[i][0] = query(1, pos);
}
lg = __lg(n);
fti(j, 1, lg)
fti(i, 1, n)
up[i][j] = up[up[i][j - 1]][j - 1];
}
int main()
{
ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
cin >> n >> m;
fti(i, 1, n)
{
cin >> t[i].F >> t[i].S;
if (t[i].F > t[i].S) t[i].S += m;
}
sort(t + 1, t + 1 + n);
res = n + 1;
int mock = t[1].F;
m -= mock;
fti(i, 1, n)
{
t[i].F -= mock, t[i].S -= mock;
}
zip();
if (n <= 20)
suf1();
else
sus2();
return 0;
}
