#include <bits/stdc++.h>
using namespace std;
#include <atcoder/all>
#include <time.h> 
using namespace atcoder;
using ll = long long;
using vll = vector<ll>;
using vvll = vector<vll>;
using vvvll = vector<vvll>;
using vb = vector<bool>;
using vvb = vector<vb>;
using vvvb = vector<vvb>;
#define all(A) A.begin(),A.end()
#define rep(i, n) for (ll i = 0; i < (ll) (n); i++)
template<class T>
bool chmax(T& p, T q, bool C = 1) {
    if (C == 0 && p == q) {
        return 1;
    }
    if (p < q) {
        p = q;
        return 1;
    }
    else {
        return 0;
    }
}
template<class T>
bool chmin(T& p, T q, bool C = 1) {
    if (C == 0 && p == q) {
        return 1;
    }
    if (p > q) {
        p = q;
        return 1;
    }
    else {
        return 0;
    }
}
ll modPow(long long a, long long n, long long p) {
    if (n == 0) return 1; // 0乗にも対応する場合
    if (n == 1) return a % p;
    if (n % 2 == 1) return (a * modPow(a, n - 1, p)) % p;
    long long t = modPow(a, n / 2, p);
    return (t * t) % p;
}
ll cnt = 0;
ll gcd(ll(a), ll(b)) {
    cnt++;
    if (a == 0)return b;
    if (b == 0)return a;
    ll c = a;
    while (a % b != 0) {
        c = a % b;
        a = b;
        b = c;
    }
    return b;
}
ll sqrtz(ll N) {
    ll L = 0;
    ll R = sqrt(N) + 10000;
    while (abs(R - L) > 1) {
        ll mid = (R + L) / 2;
        if (mid * mid <= N)L = mid;
        else R = mid;
    }
    return L;
}

using mint = modint998244353;
using vm = vector<mint>;
using vvm = vector<vm>;
using vvvm = vector<vvm>;


vector<mint> fact, factinv, inv;
const ll mod = 998244353;
void prenCkModp(ll n) {
    fact.resize(n + 5);
    factinv.resize(n + 5);
    inv.resize(n + 5);
    fact[0] = fact[1] = 1;
    factinv[0] = factinv[1] = 1;
    inv[1] = 1;
    for (ll i = 2; i < n + 5; i++) {
        fact[i] = (fact[i - 1] * i);
        inv[i] = (mod - ((inv[mod % i] * (mod / i))));
        factinv[i] = (factinv[i - 1] * inv[i]);
    }
}
mint nCk(ll n, ll k) {
    if (n < k || k < 0) return 0;
    return (fact[n] * ((factinv[k] * factinv[n - k])));
}

bool DEB = 0;

vvm mul(vvm A, vvm B) {
    ll n = A.size();
    vvm res(n, vm(n, 0));
    rep(i, n)rep(j, n)rep(k, n)res[i][j] += A[i][k] * B[k][j];
    return res;
}

using pll = pair<ll, ll>;
pair<ll, ll> op(pll a, pll b) {
    if (a.first < b.first)return a;
    else return b;
};
pll e() { return { ll(1e18),-1 }; };

int main() {

    cin.tie(nullptr);
    ios::sync_with_stdio(false);

    ll N, K;
    cin >> N >> K;
    vector<pair<ll, ll>> P(N);
    rep(i, N)cin >> P[i].second >> P[i].first;
    sort(all(P));

    ll R = -1e18;
    map<ll, ll> MP;
    ll an = 0;
    rep(i, N) {
        MP[P[i].first] = 1;
        MP[P[i].second] = 1;
        if (P[i].second >= R) {
            an++;
            chmax(R, P[i].first);
        }
    }
    ll k = 0;
    for (auto m : MP)MP[m.first] = k, k++;
    if (an < K) {
        cout << "-1" << endl;
        return 0;
    }

    segtree<pll, op, e> seg(k);
    rep(i, N)swap(P[i].first, P[i].second);
    sort(all(P));
    vvll DB(N, vll(30, -1));
    for (ll i = N - 1; i >= 0; i--) {
        ll d = MP[P[i].second];
        pll res = seg.prod(d, k);
        DB[i][0] = res.second;
        pll g = seg.get(d);
        seg.set(MP[P[i].first], op(g, { P[i].second,i }));
    }
    rep(d, 29) {
        rep(n, N) {
            if (DB[n][d] < N && DB[n][d] >= 0) {
                DB[n][d + 1] = DB[DB[n][d]][d];
            }
        }
    }
    an = 1e18;
    K--;
    rep(i, N) {
        ll x = i;
        rep(k, 30) {
            if (K & (1 << k)) {
                x = DB[x][k];
                if (x < 0 || x >= N)break;
            }
        }
        if (x < 0 || x >= N)continue;
        chmin(an, P[x].second - P[i].first);
    }
    cout << an << endl;

}