#ifndef LOCAL
#define FAST_IO
#endif

// ============
#include <algorithm>
#include <array>
#include <bitset>
#include <cassert>
#include <cmath>
#include <cstring>
#include <iomanip>
#include <iostream>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string>
#include <tuple>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>

#define OVERRIDE(a, b, c, d, ...) d
#define REP2(i, n) for (i32 i = 0; i < (i32)(n); ++i)
#define REP3(i, m, n) for (i32 i = (i32)(m); i < (i32)(n); ++i)
#define REP(...) OVERRIDE(__VA_ARGS__, REP3, REP2)(__VA_ARGS__)
#define PER(i, n) for (i32 i = (i32)(n) - 1; i >= 0; --i)
#define ALL(x) begin(x), end(x)

using namespace std;

using u32 = unsigned int;
using u64 = unsigned long long;
using i32 = signed int;
using i64 = signed long long;
using f64 = double;
using f80 = long double;

template <typename T>
using Vec = vector<T>;

template <typename T>
bool chmin(T &x, const T &y) {
    if (x > y) {
        x = y;
        return true;
    }
    return false;
}
template <typename T>
bool chmax(T &x, const T &y) {
    if (x < y) {
        x = y;
        return true;
    }
    return false;
}

#ifdef INT128

using u128 = __uint128_t;
using i128 = __int128_t;

istream &operator>>(istream &is, i128 &x) {
    i64 v;
    is >> v;
    x = v;
    return is;
}
ostream &operator<<(ostream &os, i128 x) {
    os << (i64)x;
    return os;
}
istream &operator>>(istream &is, u128 &x) {
    u64 v;
    is >> v;
    x = v;
    return is;
}
ostream &operator<<(ostream &os, u128 x) {
    os << (u64)x;
    return os;
}

#endif

[[maybe_unused]] constexpr i32 INF = 1000000100;
[[maybe_unused]] constexpr i64 INF64 = 3000000000000000100;
struct SetUpIO {
    SetUpIO() {
#ifdef FAST_IO
        ios::sync_with_stdio(false);
        cin.tie(nullptr);
#endif
        cout << fixed << setprecision(15);
    }
} set_up_io;
// ============

#ifdef DEBUGF
#else
#define DBG(x) (void) 0
#endif

int main() {
    i32 n, k;
    cin >> n >> k;
    Vec<pair<i32, i32>> rs(n);
    REP(i, n) {
        cin >> rs[i].first >> rs[i].second;
    }
    sort(ALL(rs));
    rs.emplace_back(INF, INF);
    i32 t = 20;
    Vec<Vec<i32>> db(t, Vec<i32>(n + 1, 0));
    REP(i, t) {
        db[i][n] = INF;
    }
    PER(i, n) {
        db[0][i] = min(rs[i].second, db[0][i + 1]);
    }
    REP(i, 1, t) {
        PER(j, n) {
            i32 idx = (i32)(lower_bound(ALL(rs), pair<i32, i32>(db[i - 1][j], 0)) - rs.begin());
            db[i][j] = min(db[i][j + 1], db[i - 1][idx]);
        }
    }
    i32 ans = INF;
    REP(i, n) {
        i32 cur = rs[i].first;
        REP(j, t) {
            if (k & (1 << j)) {
                i32 idx = (i32)(lower_bound(ALL(rs), pair<i32, i32>(cur, 0)) - rs.begin());
                cur = db[j][idx];
            }
        }
        if (cur != INF) {
            chmin(ans, cur - rs[i].first);
        }
    }
    if (ans == INF) {
        cout << -1 << '\n';
    } else {
        cout << ans << '\n';
    }
}