#line 1 "main.cpp"
//#pragma GCC target("avx,avx2")
//#pragma GCC optimize("O3")
//#pragma GCC optimize("unroll-loops")
#include<bits/stdc++.h>
#ifdef LOCAL
#include <debug.hpp>
#define debug(...) debug_print::multi_print(#__VA_ARGS__, __VA_ARGS__)
#else
#define debug(...) (static_cast<void>(0))
#endif
using namespace std;
using ll = long long;
using ld = long double;
using pll = pair<ll, ll>;
using pii = pair<int, int>;
using vi = vector<int>;
using vvi = vector<vi>;
using vvvi = vector<vvi>;
using vl = vector<ll>;
using vvl = vector<vl>;
using vvvl = vector<vvl>;
using vpii = vector<pii>;
using vpll = vector<pll>;
using vs = vector<string>;
template<class T> using pq = priority_queue<T, vector<T>, greater<T>>;
#define overload4(_1, _2, _3, _4, name, ...) name
#define overload3(a,b,c,name,...) name
#define rep1(n) for (ll UNUSED_NUMBER = 0; UNUSED_NUMBER < (n); ++UNUSED_NUMBER)
#define rep2(i, n) for (ll i = 0; i < (n); ++i)
#define rep3(i, a, b) for (ll i = (a); i < (b); ++i)
#define rep4(i, a, b, c) for (ll i = (a); i < (b); i += (c))
#define rep(...) overload4(__VA_ARGS__, rep4, rep3, rep2, rep1)(__VA_ARGS__)
#define rrep1(n) for(ll i = (n) - 1;i >= 0;i--)
#define rrep2(i,n) for(ll i = (n) - 1;i >= 0;i--)
#define rrep3(i,a,b) for(ll i = (b) - 1;i >= (a);i--)
#define rrep4(i,a,b,c) for(ll i = (a) + ((b)-(a)-1) / (c) * (c);i >= (a);i -= c)
#define rrep(...) overload4(__VA_ARGS__, rrep4, rrep3, rrep2, rrep1)(__VA_ARGS__)
#define all1(i) begin(i) , end(i)
#define all2(i,a) begin(i) , begin(i) + a
#define all3(i,a,b) begin(i) + a , begin(i) + b
#define all(...) overload3(__VA_ARGS__, all3, all2, all1)(__VA_ARGS__)
#define sum(...) accumulate(all(__VA_ARGS__),0LL)
template<class T> bool chmin(T &a, const T &b){ if(a > b){ a = b; return 1; } else return 0; }
template<class T> bool chmax(T &a, const T &b){ if(a < b){ a = b; return 1; } else return 0; }
template<class T> auto min(const T& a){return *min_element(all(a));}
template<class T> auto max(const T& a){return *max_element(all(a));}
template<class... Ts> void in(Ts&... t);
#define INT(...) int __VA_ARGS__; in(__VA_ARGS__)
#define LL(...) ll __VA_ARGS__; in(__VA_ARGS__)
#define STR(...) string __VA_ARGS__; in(__VA_ARGS__)
#define CHR(...) char __VA_ARGS__; in(__VA_ARGS__)
#define DBL(...) double __VA_ARGS__; in(__VA_ARGS__)
#define LD(...) ld __VA_ARGS__; in(__VA_ARGS__)
#define VEC(type, name, size) vector<type> name(size); in(name)
#define VV(type, name, h, w) vector<vector<type>> name(h, vector<type>(w)); in(name)
ll intpow(ll a, ll b){ ll ans = 1; while(b){if(b & 1) ans *= a; a *= a; b /= 2;} return ans;}
ll modpow(ll a, ll b, ll p){ ll ans = 1; a %= p;if(a < 0) a += p;while(b){ if(b & 1) (ans *= a) %= p; (a *= a) %= p; b /= 2; } return ans; }
ll GCD(ll a,ll b) { if(a == 0 || b == 0) return 0; if(a % b == 0) return b; else return GCD(b,a%b);}
ll LCM(ll a,ll b) { if(a == 0) return b; if(b == 0) return a;return a / GCD(a,b) * b;}
void Yes() {cout << "Yes\n";return;}
void No() {cout << "No\n";return;}
void YES() {cout << "YES\n";return;}
void NO() {cout << "NO\n";return;}
namespace IO{
#define VOID(a) decltype(void(a))
struct setting{ setting(){cin.tie(nullptr); ios::sync_with_stdio(false);fixed(cout); cout.precision(12);}} setting;
template<int I> struct P : P<I-1>{};
template<> struct P<0>{};
template<class T> void i(T& t){ i(t, P<3>{}); }
void i(vector<bool>::reference t, P<3>){ int a; i(a); t = a; }
template<class T> auto i(T& t, P<2>) -> VOID(cin >> t){ cin >> t; }
template<class T> auto i(T& t, P<1>) -> VOID(begin(t)){ for(auto&& x : t) i(x); }
template<class T, size_t... idx> void ituple(T& t, index_sequence<idx...>){in(get<idx>(t)...);}
template<class T> auto i(T& t, P<0>) -> VOID(tuple_size<T>{}){ituple(t, make_index_sequence<tuple_size<T>::value>{});} 
#undef VOID
}
#define unpack(a) (void)initializer_list<int>{(a, 0)...}
template<class... Ts> void in(Ts&... t){ unpack(IO :: i(t)); }

#undef unpack
static const double PI = 3.1415926535897932;
template <class F> struct REC {
    F f;
    REC(F &&f_) : f(forward<F>(f_)) {}
    template <class... Args> auto operator()(Args &&...args) const { return f(*this, forward<Args>(args)...); }};

constexpr int mod = 998244353;
//constexpr int mod = 1000000007;
#line 2 "library/graph/graph-template.hpp"
template <typename T> 
struct Edge {
	int from, to;
	T cost;
	Edge() = default;
	Edge(int _to, T _cost) : from(-1), to(_to), cost(_cost) {}
	Edge(int _from, int _to, T _cost) : from(_from), to(_to), cost(_cost) {}
	bool operator < (const Edge &a) const { return cost < a.cost; }
	bool operator > (const Edge &a) const { return cost > a.cost; }
    Edge &operator = (const int &x) {
        to = x;
        return *this;
    }
    operator int() const { return to; }
    friend ostream operator<<(ostream &os, Edge &edge) { return os << edge.to; }
};
 
template <typename T>
using Edges = vector<Edge<T>>;
template <typename T>
using Wgraph = vector<Edges<T>>;
using Ugraph = vector<vector<int>>;
Ugraph uinput(int N, int M = -1, bool is_directed = false, int origin = 1) {
    Ugraph g(N);
    if (M == -1) M = N - 1;
    while(M--) {
        int a,b;
        cin >> a >> b;
        a -= origin, b -= origin;
        g[a].push_back(b);
        if(!is_directed) g[b].push_back(a);
    }
    return g;
}
template <typename T>
Wgraph<T> winput(int N, int M = -1, bool is_directed = false,int origin = 1) {
    Wgraph<T> g(N);
    if (M == -1) M = N - 1;
    while(M--) {
        int a,b;
        T c;
        cin >> a >> b >> c;
        a -= origin, b -= origin;
        g[a].emplace_back(b,c);
        if(!is_directed) g[b].emplace_back(a,c);
    }
    return g;
}
#line 3 "library/tree/tree-query.hpp"
//1<<20頂点まで
template <typename G>
struct Tree {
    G& g;
    int root;
    vector<array<int, 20>> bl;
    vector<int> dp;
    void build() {
        bl.resize(g.size());
        dp.resize(g.size());
        for (auto &v:bl) fill(begin(v),end(v),-1);
        dfs(root, -1, 0);
    }
    void dfs(int c, int p, int _dp) {
        dp[c] = _dp;
        for (int i = p, x = 0; i != -1;) {
            bl[c][x] = i;
            i = bl[i][x], x++;
        }
        for (auto& d : g[c]) {
            if (d == p) continue;
            dfs(d, c, _dp + 1);
        }
    }
    public:
    Tree(G& _g, int _r = 0) : g(_g), root(_r) { build(); }
    int depth(int u) const { return dp[u]; }
    int par(int u) const { return u == root ? -1 : bl[u][0]; }
    int kth_ancestor(int u, int k) const {
        if (dp[u] < k) return -1;
        while (k) {
        int t = __builtin_ctz(k);
        u = bl[u][t], k ^= 1 << t;
        }
        return u;
    }
    int nxt(int s, int t) const {
        if (dp[s] >= dp[t]) return par(s);
        int u = kth_ancestor(t, dp[t] - dp[s] - 1);
        return bl[u][0] == s ? u : bl[s][0];
    }
    vector<int> path(int s, int t) const {
        vector<int> pre, suf;
        while (dp[s] > dp[t]) {
            pre.push_back(s);
            s = bl[s][0];
        }
        while (dp[s] < dp[t]) {
            suf.push_back(t);
            t = bl[t][0];
        }
        while (s != t) {
            pre.push_back(s);
            suf.push_back(t);
            s = bl[s][0];
            t = bl[t][0];
        }
        pre.push_back(s);
        reverse(begin(suf), end(suf));
        copy(begin(suf), end(suf), back_inserter(pre));
        return pre;
    }
    int lca(int u, int v) {
        if (dp[u] != dp[v]) {
            if (dp[u] > dp[v]) swap(u, v);
            v = kth_ancestor(v, dp[v] - dp[u]);
        }
        if (u == v) return u;
        for (int i = __lg(dp[u]); i >= 0; --i) {
            if (dp[u] < (1 << i)) continue;
            if (bl[u][i] != bl[v][i]) u = bl[u][i], v = bl[v][i];
        }
        return bl[u][0];
    }
    //st-gl間より長い距離をjumpしようとすると-1が返ってくる
    int jump(int st,int gl,int distance) const {
        int ancestor = lca(st,gl);
        int path_distance = dp[st] + dp[gl] - 2 * dp[ancestor];
        if(path_distance < distance) return -1;
        if(dp[st] - dp[ancestor] >= distance) return kth_ancestor(st,distance);
        return kth_ancestor(gl,path_distance - distance);
    }
};
#line 90 "main.cpp"
int main() {
    INT(n,k);
    vi L(n),R(n);
    rep(i,n) cin >> L[i] >> R[i];
    vi ord(n);
    iota(all(ord),0);
    sort(all(ord),[&](int x,int y){return R[x] < R[y];});
    queue<int> q;
    vvi g(n+1);
    rep(i,n) {
        while(!q.empty() && R[ord[q.front()]] <= L[ord[i]]) {
            int pre = q.front();
            q.pop();
            g[ord[pre]].emplace_back(ord[i]);
            g[ord[i]].emplace_back(ord[pre]);
        }
        q.emplace(i);
    }
    while(!q.empty()) {
        int pre = q.front();
        q.pop();
        g[ord[pre]].emplace_back(n);
        g[n].emplace_back(ord[pre]);
    }
    debug(g);
    Tree<vvi> tree(g,n);
    int ans = 2e9;
    rep(i,n) {
        int anc = tree.kth_ancestor(i,k-1);
        if(anc == n || anc == -1) continue;
        chmin(ans,R[anc] - L[i]);
    }
    if(ans == (int)2e9) cout << -1 << '\n';
    else cout << ans << '\n';
}