#include <bits/stdc++.h>
using namespace std;
using P = pair<int, int>;

int main(){
    int N, K;
    cin >> N >> K;
    vector<int> L(N), R(N);
    vector<P> v(N);
    for(int i = 0; i < N; i++){
        cin >> L[i] >> R[i];
        v[i] = P(L[i], R[i]);
    }
    sort(v.begin(), v.end());
    vector<P> w;
    for(int idx = 0; auto [l, r] : v){
        w.emplace_back(l, idx);
        w.emplace_back(r, -(idx + 1));
        idx++;
    }
    sort(w.begin(), w.end());
    const int B = 20;
    vector<vector<int>> doubling(N, vector<int>(B, -1));
    priority_queue<P, vector<P>, greater<P>> pq;
    for(int i = (int)w.size() - 1; i >= 0; i--){
        auto [t, idx] = w[i];
        if(idx >= 0){
            pq.emplace(v[idx].second, idx);
        }else if(!pq.empty()){
            auto [tt, tidx] = pq.top();
            doubling[-1 * idx - 1][0] = tidx;
        }
    }
    for(int i = 0; i < B - 1; i++){
        for(int j = 0; j < N; j++){
            if(doubling[j][i] == -1) continue;
            doubling[j][i + 1] = doubling[doubling[j][i]][i];
        }
    }
    const int inf = 1'000'000'100; // 1e9 + 100
    int ans = inf;
    for(int i = 0; i < N; i++){
        int cur = i;
        for(int j = 0; j < B; j++){
            if(cur == -1) break;
            if(((K - 1) >> j) & 1) cur = doubling[cur][j];
        }
        if(cur != -1) ans = min(ans, v[cur].second - v[i].first);
    }
    if(ans != inf) cout << ans << endl;
    else cout << -1 << endl;
}