#include <bits/stdc++.h>

using namespace std;
using ll = long long;

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

    ll N, K, L, R, r=1, ans=1e18;
    cin >> N >> K;
    K--;
    vector<pair<ll, ll>> RL(N+1);

    for (int i=1; i<=N; i++){
        cin >> L >> R;
        RL[i] = {R, L};
    }
    if (K == 1){
        for (auto [R, L] : RL) ans = min(ans, R-L);
        cout << ans << endl;
        return 0;
    }
    sort(RL.begin(), RL.end());
    /*
       f(i) = iのミーティングに参加したとき、次に参加するミーティング
       R_i<=L_jかつR_jが最小のものを選ぶ
       f^(K-1)(i)<=NならOK
    */

    vector<ll> dp(N+2), v(N+2);
    for (int l=1; l<=N; l++){
        while(r <= N && RL[l].first > RL[r].second) r++;
        dp[l] = r;
    }
    iota(v.begin(), v.end(), 0);
    dp[N+1] = N+1;
    //for (int i=1; i<=N; i++) cout << dp[i] << " ";
    //cout << endl;
    while(K){
        if (K&1){
            for (int i=1; i<=N; i++) v[i] = v[dp[i]];
        }
        vector<ll> pd(N+2);
        for (int i=1; i<=N; i++) pd[i] = dp[dp[i]];
        swap(dp, pd);
        K >>= 1;
    }

    for (int i=1; i<=N; i++){
        if (v[i] <= N){
            ans = min(ans, RL[v[i]].first-RL[i].second);
        }
    }

    cout << (ans != 1e18 ? ans : -1) << endl;

    return 0;
}