#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
const int N=602;
int dp[N][N][N];
int s[N],e[N],mpe[N],mps[N],ord[N];
vector<int> s_p;
bool funot(int a,int b)
{
return (s[a]<s[b] or (s[a]==s[b] and e[a]<e[b]));
}
int main()
{
ios::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
int n,m;
cin>>n>>m;
for(int i=0;i<n;i++)
{
ord[i]=i;
cin>>s[i]>>e[i];
s_p.push_back(s[i]);
s_p.push_back(e[i]);
// if(s[i]>0)
// s_p.push_back(s[i]-1);
// if((s[i]+1)<m)
// s_p.push_back(s[i]+1);
// if((e[i]+1)<m)
// s_p.push_back(e[i]+1);
// if(e[i]>0)
// s_p.push_back(e[i]-1);
}
sort(ord,ord+n,funot);
s_p.push_back(0);
s_p.push_back(m-1);
if(n<=500)
{
sort(begin(s_p),end(s_p));
s_p.resize(unique(begin(s_p),end(s_p))-begin(s_p));
int sz=s_p.size();
for(int i=0;i<=n;i++)
{
if(i<n)
{
mps[i]=lower_bound(begin(s_p),end(s_p),s[i])-begin(s_p);
mpe[i]=lower_bound(begin(s_p),end(s_p),e[i])-begin(s_p);
}
for(int j=0;j<sz;j++)
{
for(int k=0;k<sz;k++)
{
dp[i][j][k]=n+1e5;
}
}
}
int ans=n+1e5;
for(int iads=0;iads<n;iads++)
{
int i=ord[iads];
for(int l=sz-1;l>=1;l--)
{
for(int j=0;j<sz;j++)
{
int k=j+l-1;
if(k<sz)
{
if((j-1)>=0)
dp[iads][j][k]=min(dp[iads][j][k],dp[iads][j-1][k]);
if((k+1)<sz)
dp[iads][j][k]=min(dp[iads][j][k],dp[iads][j][k+1]);
if((k+1)<sz and (j-1)>=0)
dp[iads][j][k]=min(dp[iads][j][k],dp[iads][j-1][k+1]);
}
}
}
for(int j=0;j<sz;j++)
for(int k=j;k<sz;k++)
dp[iads+1][j][k]=min(dp[iads+1][j][k],dp[iads][j][k]);
if(s[i]<=e[i])
{
for(int j=0;j<sz;j++)
{
for(int k=j;k<sz;k++)
{
int l=s_p[j],r=s_p[k];
if(s[i]<=l and r<=e[i])
{
// can be covered using only current meaning i
dp[iads+1][j][k]=min(dp[iads+1][j][k],1);
}
if(s[i]<=r)
dp[iads+1][j][max(k,mpe[i])]=min(dp[iads+1][j][max(k,mpe[i])],dp[iads][j][k]+1);
}
}
}
else
{
for(int j=0;j<sz;j++)
{
for(int k=j;k<sz;k++)
{
int l=s_p[j],r=s_p[k];
if(s[i]<=l)
{
// simply it is two segments [0,e[i]] and [s[i],m]
ans=min(ans,dp[iads][mpe[i]][mps[i]]+1);
//
// can be covered using only current meaning i
dp[iads+1][j][k]=min(dp[iads+1][j][k],1);
}
if(r<=e[i])
dp[iads+1][j][k]=min(dp[iads+1][j][k],1);
if(s[i]<=r)
{
// cover [l,r]
// we can do [r,m]
// [l,m]
// s[i] to m
dp[iads+1][j][sz-1]=min(dp[iads+1][j][sz-1],dp[iads][j][k]+1);
}
if(l<=e[i])
{
// cover [l,r]
// we can do [0,l]
// [0,r]
dp[iads+1][0][k]=min(dp[iads+1][0][k],dp[iads][j][k]+1);
}
if(l<=e[i] and s[i]<=r)
{
dp[iads+1][0][sz-1]=min(dp[iads+1][0][sz-1],dp[iads][j][k]+1);
ans=min(ans,dp[iads][j][k]+1);
}
}
}
}
}
ans=min(ans,dp[n][0][sz-1]);
if(ans>n)
ans=-1;
cout<<ans<<endl;
}
else
{
cout<<-1<<endl;
}
}
