#include <bits/stdc++.h>
using namespace std;
#include <atcoder/all>
using namespace atcoder;
#define rep(i, n) for(int i=0;i<(n);++i)
#define rep1(i, n) for(int i=1;i<=(n);i++)
#define ll long long
using mint = modint998244353;
using P = pair<ll,ll>;
using lb = long double;
using T = tuple<ll, ll, ll>;
#ifdef LOCAL
#  include <debug_print.hpp>
#  define dbg(...) debug_print::multi_print(#__VA_ARGS__, __VA_ARGS__)
#else
#  define dbg(...) (static_cast<void>(0))
#endif

// Coodinate Compression
// https://youtu.be/fR3W5IcBGLQ?t=8550
template<typename T=int>
struct CC {
  bool initialized;
  vector<T> xs;
  CC(): initialized(false) {}
  void add(T x) { xs.push_back(x);}
  void init() {
    sort(xs.begin(), xs.end());
    xs.erase(unique(xs.begin(),xs.end()),xs.end());
    initialized = true;
  }
  int operator()(T x) {
    if (!initialized) init();
    return upper_bound(xs.begin(), xs.end(), x) - xs.begin() - 1;
  }
  T operator[](int i) {
    if (!initialized) init();
    return xs[i];
  }
  int size() {
    if (!initialized) init();
    return xs.size();
  }
};

int op(int a, int b) {
    return min(a,b);
}

int e(){
    return 1e9;
}

int main()
{
    int n, k;
    cin >> n >> k;
    vector<pair<int,int>> l(n);
    rep(i,n) cin >> l[i].first >> l[i].second;
    CC cc;
    rep(i,n) {
        cc.add(l[i].first);
        cc.add(l[i].second);
    }
    int m = cc.size();
    segtree<int, op, e> seg(m);
    rep(i,n){
        seg.set(cc(l[i].first), min(seg.get(cc(l[i].first)), cc(l[i].second)));
    }
    vector<vector<int>> dp(31, vector<int>(m, -1));
    rep(i,n){
        dp[0][cc(l[i].first)] = seg.prod(cc(l[i].first), m);
        if(dp[0][cc(l[i].second)]==1e9) dp[0][cc(l[i].second)] = -1;
        dp[0][cc(l[i].second)] = seg.prod(cc(l[i].second), m);
        if(dp[0][cc(l[i].second)]==1e9) dp[0][cc(l[i].second)] = -1;
    }
    dbg(dp);
    rep(k,30){
        rep(j,m){
            if(dp[k][j]==-1) continue;
            dp[k+1][j] = dp[k][dp[k][j]];
        }
    }
    int ans = 2e9;
    dbg(dp);
    rep(i,n){
        int now = cc(l[i].first);
        dbg(now);
        dbg(k);
        rep(j,30){
            if(k>>j&1) {
                if(now==-1) continue;
                now = dp[j][now];
                dbg(now);
                dbg(j);
            }
        }
        dbg(1,now);
        if(now==-1) continue;
        ans = min(ans, cc[now]-l[i].first);
    }
    if(ans==2e9) ans = -1;
    cout << ans << endl;
    return 0;
}