#include <iostream>
#include <algorithm>
#include <iomanip>
#include <vector>
#include <queue>
#include <deque>
#include <set>
#include <map>
#include <tuple>
#include <cmath>
#include <numeric>
#include <functional>
#include <cassert>

#define debug_value(x) cerr << "line" << __LINE__ << ":<" << __func__ << ">:" << #x << "=" << x << endl;
#define debug(x) cerr << "line" << __LINE__ << ":<" << __func__ << ">:" << x << endl;

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; }

using namespace std;
typedef long long ll;

template<typename T>
vector<vector<T>> vec2d(int n, int m, T v){
    return vector<vector<T>>(n, vector<T>(m, v));
}

template<typename T>
vector<vector<vector<T>>> vec3d(int n, int m, int k, T v){
    return vector<vector<vector<T>>>(n, vector<vector<T>>(m, vector<T>(k, v)));
}

template<typename T>
void print_vector(vector<T> v, char delimiter=' '){
    if(v.empty()) {
        cout << endl;
        return;
    }
    for(int i = 0; i+1 < v.size(); i++) cout << v[i] << delimiter;
    cout << v.back() << endl;
}

template<typename T>
class Compress{
    public:
    vector<T> data;
    int offset;
    Compress(vector<T> data_, int offset=0): offset(offset){
        data = data_;
        sort(begin(data), end(data));
        data.erase(unique(begin(data), end(data)), end(data));
    };
    int operator[](T x) {
        auto p = lower_bound(data.begin(), data.end(), x);
        assert(x == *p);
        return offset+(p-data.begin());
	}
    T inv(int x){
        return data[x-offset];
    }
    int size(){
        return data.size();
    }
};

const int inf = 1e9;

int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout << setprecision(10) << fixed;
    int n, k; cin >> n >> k;
    vector<int> l(n), r(n);
    for(int i = 0; i < n; i++) {
        cin >> l[i] >> r[i];
    }
    vector<int> v = l;
    for(int i = 0; i < n; i++) v.push_back(r[i]);
    auto cp = Compress<int>(v);
    int m = cp.size();
    vector<int> right_min(m, inf);
    auto nx = vec2d(m, 20, inf);
    for(int i = 0; i < n; i++) {
        chmin(nx[cp[l[i]]][0], cp[r[i]]);
    }
    for(int i = m-2; i >= 0; i--){
        chmin(nx[i][0], nx[i+1][0]);
    }
    for(int j = 1; j < 20; j++){
        for(int i = 0; i < m; i++){
            if(nx[i][j-1] != inf){
                nx[i][j] = nx[nx[i][j-1]][j-1];
            }
        }
        for(int i = m-2; i >= 0; i--){
            chmin(nx[i][j], nx[i+1][j]);
        }
    }
    
    auto go = [&](int i, int cnt){
        int ans = i;
        for(int j = 0; j < 20; j++){
            if(cnt&(1<<j)){
                ans = nx[ans][j];
            }
            if(ans == inf) return ans;
        }
        return ans;
    };
    int ans = -1;
    for(int i = 0; i < m; i++){
        int x = go(i, k);
        if(x == inf) continue;
        int len = cp.data[x]-cp.data[i];
        if(ans == -1){
            ans = len;
        }else{
            chmin(ans, len);
        }
    }
    cout << ans << endl;
}