//*
#pragma GCC target("avx2")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
//*/

#include <bits/stdc++.h>

// #include <atcoder/all>
// #include <atcoder/lazysegtree>

using namespace std;
// using namespace atcoder;

// #define _GLIBCXX_DEBUG

#define DEBUG(x) cerr << #x << ": " << x << endl;
#define DEBUG_VEC(v)                                        \
    cerr << #v << ":";                                      \
    for (int iiiiiiii = 0; iiiiiiii < v.size(); iiiiiiii++) \
        cerr << " " << v[iiiiiiii];                         \
    cerr << endl;
#define DEBUG_MAT(v)                                \
    cerr << #v << endl;                             \
    for (int iv = 0; iv < v.size(); iv++) {         \
        for (int jv = 0; jv < v[iv].size(); jv++) { \
            cerr << v[iv][jv] << " ";               \
        }                                           \
        cerr << endl;                               \
    }
typedef long long ll;
// #define int ll

#define vi vector<int>
#define vl vector<ll>
#define vii vector<vector<int>>
#define vll vector<vector<ll>>
#define vs vector<string>
#define pii pair<int, int>
#define pis pair<int, string>
#define psi pair<string, int>
#define pll pair<ll, ll>
template <class S, class T>
pair<S, T> operator+(const pair<S, T> &s, const pair<S, T> &t) {
    return pair<S, T>(s.first + t.first, s.second + t.second);
}
template <class S, class T>
pair<S, T> operator-(const pair<S, T> &s, const pair<S, T> &t) { return pair<S, T>(s.first - t.first, s.second - t.second); }
template <class S, class T>
ostream &operator<<(ostream &os, pair<S, T> p) {
    os << "(" << p.first << ", " << p.second << ")";
    return os;
}
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define rep1(i, n) for (int i = 1; i <= (int)(n); i++)
#define rrep(i, n) for (int i = (int)(n)-1; i >= 0; i--)
#define rrep1(i, n) for (int i = (int)(n); i > 0; i--)
#define REP(i, a, b) for (int i = a; i < b; i++)
#define in(x, a, b) (a <= x && x < b)
#define all(c) c.begin(), c.end()
void YES(bool t = true) {
    cout << (t ? "YES" : "NO") << endl;
}
void Yes(bool t = true) { cout << (t ? "Yes" : "No") << endl; }
void yes(bool t = true) { cout << (t ? "yes" : "no") << endl; }
void NO(bool t = true) { cout << (t ? "NO" : "YES") << endl; }
void No(bool t = true) { cout << (t ? "No" : "Yes") << endl; }
void no(bool t = true) { cout << (t ? "no" : "yes") << endl; }
template <class T>
bool chmax(T &a, const T &b) {
    if (a < b) {
        a = b;
        return 1;
    }
    return 0;
}
template <class T>
bool chmin(T &a, const T &b) {
    if (a > b) {
        a = b;
        return 1;
    }
    return 0;
}
#define UNIQUE(v) v.erase(std::unique(v.begin(), v.end()), v.end());
const ll inf = 1000000001;
const ll INF = (ll)1e18 + 1;
const long double pi = 3.1415926535897932384626433832795028841971L;
int popcount(ll t) { return __builtin_popcountll(t); }
// int dx[4] = {1, 0, -1, 0}, dy[4] = {0, 1, 0, -1};
// int dx2[8] = { 1,1,0,-1,-1,-1,0,1 }, dy2[8] = { 0,1,1,1,0,-1,-1,-1 };
vi dx = {0, 0, -1, 1}, dy = {-1, 1, 0, 0};
vi dx2 = {1, 1, 0, -1, -1, -1, 0, 1}, dy2 = {0, 1, 1, 1, 0, -1, -1, -1};
struct Setup_io {
    Setup_io() {
        ios_base::sync_with_stdio(0), cin.tie(0), cout.tie(0);
        cout << fixed << setprecision(25);
    }
} setup_io;
// const ll MOD = 1000000007;
const ll MOD = 998244353;
// #define mp make_pair
// #define endl '\n'

template <typename Semigroup>
struct DisjointSparseTable {
    using F = function<Semigroup(Semigroup, Semigroup)>;
    const F f;
    vector<vector<Semigroup>> st;

    DisjointSparseTable(const vector<Semigroup> v, const F f) : f(f) {

        int b = 0;
        while ((1 << b) <= v.size())
            ++b;
        st.resize(b, vector<Semigroup>(v.size(), Semigroup()));
        for (int i = 0; i < v.size(); i++)
            st[0][i] = v[i];
        for (int i = 1; i < b; i++) {
            int shift = 1 << i;
            for (int j = 0; j < v.size(); j += shift << 1) {
                int t = min(j + shift, (int)v.size());
                st[i][t - 1] = v[t - 1];
                for (int k = t - 2; k >= j; k--)
                    st[i][k] = f(v[k], st[i][k + 1]);
                if (v.size() <= t) break;
                st[i][t] = v[t];
                int r = min(t + shift, (int)v.size());
                for (int k = t + 1; k < r; k++)
                    st[i][k] = f(st[i][k - 1], v[k]);
            }
        }
    }

    Semigroup query(int l, int r) {
        if (l >= --r) return st[0][l];
        int p = 31 - __builtin_clz(l ^ r);
        return f(st[p][l], st[p][r]);
    }
};

pll f(pll a, pll b) {
    return min(a, b);
}

signed main() {
    int n, k;
    cin >> n >> k;

    vector<pll> lr(n);
    rep(i, n) {
        cin >> lr[i].first >> lr[i].second;
    }
    sort(all(lr));

    vector<pll> ini(n);
    rep(i, n) {
        ini[i] = pll(lr[i].second, i);
    }
    DisjointSparseTable<pll> dst(ini, f);

    constexpr int K = 21;
    vector dp(K, vector(n, pii(-1, -1)));
    rep(i, n) {
        int r = lr[i].second;
        int idx = lower_bound(all(lr), pll(r, -1)) - lr.begin();
        if (idx == n) {
            dp[0][i] = pii(r, inf);
        } else {
            int mi = dst.query(idx, n).second;
            dp[0][i] = pii(r, mi);
        }
    }

    rep(ki, K - 2) {
        rep(i, n) {
            int r = dp[ki][i].first;
            int idx = dp[ki][i].second;
            if (idx == inf) {
                dp[ki + 1][i] = pii(inf, inf);
                continue;
            }
            dp[ki + 1][i] = dp[ki][idx];
        }
    }

    ll ans = inf;
    rep(i, n) {
        int rem = k;
        int now = i;
        int last_r = inf;
        int k = K - 1;
        while (rem > 0) {
            if (now == inf) {
                break;
            }
            while (rem < (1 << k)) {
                k--;
            }
            if (dp[k][now].first == inf) {
                break;
            }
            last_r = dp[k][now].first;
            now = dp[k][now].second;
            rem -= (1 << k);
        }

        if (rem == 0) {
            chmin(ans, last_r - lr[i].first);
        }
    }

    if (ans == inf) {
        cout << -1 << endl;
    } else {
        cout << ans << endl;
    }
}