#include <bits/stdc++.h>
#define ff first
#define ss second
#define int long long
#define pii pair<int, int>
#define ff first
#define ss second
using namespace std;
const int inf = 1e17;
const int mod = 998244353;
int solve2(int n,int m,vector<pii> v)
{
for (int i = 0; i< n; i++)
{
while(v[i].ff>v[i].ss)
v[i].ss+=m;
}
sort(v.begin(),v.end());
{
vector<bool> del(n);
int maxi = 0;
for (int i = 0; i < n;i ++)
{
if (v[i].ss <= maxi)
del[i] = 1;
maxi = max(maxi,v[i].ss);
}
vector<pii> neu;
for (int i = 0; i < n; i++)
if (!del[i])
neu.push_back(v[i]);
n = neu.size();
v = neu;
}
{
vector<bool> del(n);
int mini = v[n-1].ss;
for (int i = n-1; i >= 0;i--)
{
if (v[i].ff >= mini)
del[i] = 1;
mini = min(mini,v[i].ff);
}
vector<pii> neu;
for (int i = 0; i < n; i++)
if (!del[i])
neu.push_back(v[i]);
n = neu.size();
v = neu;
}
for (int i = 0; i< n;i++)
v[i].ss%=m;
/*for (int i = 0; i < n; i++)
cout << v[i].ff << ' ' << v[i].ss << '\n';*/
int prev = v[0].ss;
int used = 1;
if (v[0].ff != 0)
{
return -1;
}
for (int i = 1; i < n; i++)
{
if (v[i].ff > prev)
{
used++;
prev = v[i-1].ss;
if (v[i].ff > prev)
{
return -1;
}
}
if (prev != 0 && v[i].ss == 0)
{
used++;
prev = inf;
break;
}
}
if(prev == inf)
return used;
else
return -1;
}
signed main()
{
ios_base::sync_with_stdio(false);
cin.tie(0);
cout.tie(0);
int n,m;
cin >> n >> m;
vector<pii> v(n);
for (int i = 0; i < n; i++)
cin >> v[i].ff >> v[i].ss;
cout << solve2(n,m,v) << endl;
}
