#include <bits/stdc++.h>

using namespace std;

#define overload4(_1, _2, _3, _4, name, ...) name
#define rep1(n) for(int i = 0; i < (int)(n); ++i)
#define rep2(i, n) for(int i = 0; i < (int)(n); ++i)
#define rep3(i, a, b) for(int i = (a); i < (int)(b); ++i)
#define rep4(i, a, b, c) for(int i = (a); i < (int)(b); i += (c))
#define rep(...) overload4(__VA_ARGS__, rep4, rep3, rep2, rep1)(__VA_ARGS__)

#define rrep(i,n) for(int i = (int)(n) - 1; i >= 0; --i)
#define ALL(a) (a).begin(), (a).end()
#define Sort(a) (sort((a).begin(), (a).end()))
#define RSort(a) (sort((a).rbegin(), (a).rend()))
#define UNIQUE(a) (a.erase(unique((a).begin(), (a).end()), (a).end()))

typedef long long int ll;
typedef unsigned long long ul;
typedef long double ld;
typedef vector<int> vi;
typedef vector<long long> vll;
typedef vector<char> vc;
typedef vector<string> vst;
typedef vector<double> vd;
typedef vector<long double> vld;
typedef pair<long long, long long> P;

template<class T> long long sum(const T& a){ return accumulate(a.begin(), a.end(), 0LL); }
template<class T> auto min(const T& a){ return *min_element(a.begin(), a.end()); }
template<class T> auto max(const T& a){ return *max_element(a.begin(), a.end()); }

const long long MINF = 0x7fffffffffff;
const long long INF = 0x1fffffffffffffff;
const long long MOD = 998244353;
const long double EPS = 1e-9;
const long double PI = acos(-1);
 
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }

template<typename T1, typename T2> istream &operator>>(istream &is, pair<T1, T2> &p){ is >> p.first >> p.second; return is; }
template<typename T1, typename T2> ostream &operator<<(ostream &os, const pair<T1, T2> &p){ os << "(" << p.first << ", " << p.second << ")"; return os; }
template<typename T> istream &operator>>(istream &is, vector<T> &v){ for(T &in : v) is >> in; return is; }
template<typename T> ostream &operator<<(ostream &os, const vector<T> &v){ for(int i = 0; i < (int) v.size(); ++i){ os << v[i] << (i + 1 != (int) v.size() ? " " : ""); } return os; }
template <typename T, typename S> ostream &operator<<(ostream &os, const map<T, S> &mp){ for(auto &[key, val] : mp){ os << key << ":" << val << " "; } return os; }
template <typename T> ostream &operator<<(ostream &os, const set<T> &st){ auto itr = st.begin(); for(int i = 0; i < (int)st.size(); ++i){ os << *itr << (i + 1 != (int)st.size() ? " " : ""); itr++; } return os; }
template <typename T> ostream &operator<<(ostream &os, const multiset<T> &st){ auto itr = st.begin(); for(int i = 0; i < (int)st.size(); ++i){ os << *itr << (i + 1 != (int)st.size() ? " " : ""); itr++; } return os; }
template <typename T> ostream &operator<<(ostream &os, queue<T> q){ while(q.size()){ os << q.front() << " "; q.pop(); } return os; }
template <typename T> ostream &operator<<(ostream &os, deque<T> q){ while(q.size()){ os << q.front() << " "; q.pop_front(); } return os; }
template <typename T> ostream &operator<<(ostream &os, stack<T> st){ while(st.size()){ os << st.top() << " "; st.pop(); } return os; }
template <class T, class Container, class Compare> ostream &operator<<(ostream &os, priority_queue<T, Container, Compare> pq){ while(pq.size()){ os << pq.top() << " "; pq.pop(); } return os; }

template<class T, class U> inline T vin(T& vec, U n) { vec.resize(n); for(int i = 0; i < (int) n; ++i) cin >> vec[i]; return vec; }
template<class T> inline void vout(T vec, string s = "\n"){ for(auto x : vec) cout << x << s; }
template<class... T> void in(T&... a){ (cin >> ... >> a); }
void out(){ cout << '\n'; }
template<class T, class... Ts> void out(const T& a, const Ts&... b){ cout << a; (cout << ... << (cout << ' ', b)); cout << '\n'; }
template<class T, class U> void inGraph(vector<vector<T>>& G, U n, U m, bool directed = false){ G.resize(n); for(int i = 0; i < m; ++i){ int a, b; cin >> a >> b; a--, b--; G[a].push_back(b); if(!directed) G[b].push_back(a); } }

template <typename X>
struct SegTree{
    using FX = function<X(X, X)>; // X•X -> X となる関数の型
    int n;
    FX fx;
    const X ex;
    vector<X> dat;

    SegTree(int n_, const FX &fx_, const X &ex_) : n(), fx(fx_), ex(ex_){
        int x = 1;
        while(n_ > x){
            x *= 2;
        }
        n = x;
        dat.assign(n * 2, ex);
    }

    X get(int i) const {
        return dat[i + n];
    }
    
    void set(int i, X x){ dat[i + n] = x; }

    void build(){
        for(int k = n - 1; k >= 1; k--) dat[k] = fx(dat[k * 2], dat[k * 2 + 1]);
    }

    void update(int i, X x){
        i += n;
        dat[i] = x;
        while(i > 0){
            i >>= 1;  // parent
            dat[i] = fx(dat[i * 2], dat[i * 2 + 1]);
        }
    }

    X query(int a, int b){
        X vl = ex;
        X vr = ex;
        int l = a + n;
        int r = b + n;
        while(l < r){
            if(l & 1) vl = fx(vl, dat[l++]);
            if(r & 1) vr = fx(dat[--r], vr);
            l >>= 1;
            r >>= 1;
        }
        return fx(vl, vr);
    }
    
    X operator [](int i) const {
        return dat[i + n];
    }
};

template <typename T>
struct compress{
    vector<T> sorted, compressed;

    compress(const vector<T>& vec){
        int n = vec.size();
        compressed.resize(n);
        for(T x : vec){
            sorted.emplace_back(x);
        }
        sort(sorted.begin(), sorted.end());
        sorted.erase(unique(sorted.begin(), sorted.end()), sorted.end());
        for(int i = 0; i < n; ++i){
            compressed[i] = lower_bound(sorted.begin(), sorted.end(), vec[i]) - sorted.begin();
        }
    }

    int get(const T& x) const{
        return lower_bound(sorted.begin(), sorted.end(), x) - sorted.begin();
    }

    T inv(const T& x) const{
        return sorted[x];
    }

    size_t size() const{
        return sorted.size();
    }

    vector<T> getCompressed() const{
        return compressed;
    }
};

ll n, k;
vector<P> lr;

void input(){
    in(n, k);
    lr.resize(n);
    rep(i, n){
        in(lr[i].first, lr[i].second);
    }
}

void solve(){
    Sort(lr);
    map<ll, ll> mp;
    ll mn = INF, mnK = -1;
    rrep(i, n){
        auto [l, r] = lr[i];
        if(chmin(mn, r)){
            mnK = i;
        }
        mp[l] = mnK;
    }

    ll logK = 20;
    vector<vll> doubling(logK, vll(20, -1));
    rep(i, n){
        auto iter = mp.lower_bound(lr[i].second);
        if(iter == mp.end()){
            doubling[0][i] = -1;
        }else{
            doubling[0][i] = iter->second;
        }
    }

    rep(j, logK - 1){
        rep(i, n){
        	if(doubling[j][i] == -1) continue;
            doubling[j + 1][i] = doubling[j][doubling[j][i]];
        }
    }

    ll ans = INF;
    rep(i, n){
        ll now = i;
        ll nowK = k - 1;
        for(int j = 0; nowK > 0 && now != -1; j++){
            if(nowK & 1) now = doubling[j][now];
            nowK = nowK >> 1;
        }
        if(now != -1){
            chmin(ans, lr[now].second - lr[i].first);
        }
    }

    if(ans == INF){
        out(-1);
    }else{
        out(ans);
    }
}

int main(){
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cout << fixed << setprecision(20);
    
    input();
    solve();
}