#include<algorithm>
#include<iostream>
#include<vector>
#include<map>
#include<cassert>
#include<set>
#include<queue>
#include<cstdint>
#include<unordered_map>
using namespace std;
using ll = long long;
#include<stack>

int main(){
    cin.tie(nullptr);
    ios::sync_with_stdio(false);

    int k,n;
    cin>>n>>k;
    vector<ll> l(n),r(n);
    for(int i = 0;i<n;i++) cin>>l[i]>>r[i];
    vector<int> idx(n);
    for(int i = 0;i<n;i++) idx[i] = i;
    sort(idx.begin(),idx.end(),[&](int i,int j){
        return r[i] < r[j];});
    vector<pair<ll,ll>> use;
    for(int i = 0;i<n;i++){
        int ni = idx[i];
        if(!use.empty()&&use.back().first>=l[ni]) continue;
        use.push_back(make_pair(l[ni],i));
    }
    const int B = 40;
    vector<vector<ll>> dp(n,vector<ll>(B+1,2e9));
    for(int i = 0;i<n;i++){
        dp[i][0] = r[idx[i]];
    }
    for(int i = 1;i<=B;i++){
        for(int j = 0;j<n;j++){
            ll now = dp[j][i-1];
            if(now==2e9) continue;
            int ni = lower_bound(use.begin(),use.end(),make_pair(now,(ll)-1)) - use.begin();
            if(ni==use.size()) dp[j][i] = (ll)2e9;
            else{
                ll nj = use[ni].second;
                dp[j][i] = dp[nj][i-1];
            }
        }
    }
    ll ans = 1e18;
    for(int i = 0;i<n;i++){
        ll want = k;
        ll ni = l[idx[i]];
        for(int j = 0;j<=B;j++){
            if(~want>>j&1) continue;
            int nj = lower_bound(use.begin(),use.end(),make_pair(ni,(ll)-1)) - use.begin();
            if(nj==use.size()) {
                ni = 2e9;
                break;
            }
            ni = use[nj].second;
            ni = dp[ni][j];
            if(ni==2e9) break;
        }
        if(ni==2e9) continue;
        ans = min(ans,ni-l[idx[i]]);
    }
    if(ans==1e18) ans = -1;
    cout<<ans<<endl;
}