#include<bits/stdc++.h>
using namespace std;
#define P pair<int,int>
P l[100001];
int ls=INT_MAX,hs=0,n;
long long m;
int main()
{
    freopen("double_muscle.in","r",stdin);
    freopen("double_muscle.out","w",stdout);
    cin>>n>>m;
    for(int i=1;i<=n;++i)
    {
        int a,b;
        cin>>a>>b;
        if(l[a].first==0) l[a].first=b;
        if(b>l[a].second) l[a].second=b;
        if(b<l[a].first)  l[a].first=b;
        if(b>hs) hs=b;
        if(b<ls) ls=b;
    }
    if(hs-ls<m)
    {
        cout<<-1;
        return 0;
    }
    return 0;
}