#include <bits/stdc++.h>
using namespace std;
using i64 = int64_t;
const i64 INF = LLONG_MAX / 4;
void chmin(i64& a, i64 b) { if(a > b) a = b; }


int main() {
    cin.tie(0)->sync_with_stdio(0);
    
    i64 N, K;
    cin >> N >> K;
    vector<pair<i64, i64>> A(N);
    map<i64, i64> nx_end;
    for(auto& [L, R] : A) {
        cin >> L >> R;
        chmin(nx_end.try_emplace(L, R).first->second, R);
    }
    {
        i64 mn = INF;
        for(auto& [key, val] : nx_end | views::reverse) {
            chmin(mn, val);
            val = mn;
        }
    }
    map<i64, i64> nx[18];
    nx[0] = nx_end;
    for(i64 l = 0; l + 1 < 18; l++) {
        auto p = nx[l].begin();
        for(auto [key, val] : nx[l]) {
            while(p != nx[l].end() && p->first < val) p++;
            if(p == nx[l].end()) break;
            nx[l + 1].emplace_hint(nx[l + 1].end(), key, p->second);
        }
    }
    
    i64 ans = INF;
    for(const i64 L : nx[0] | views::keys) {
        i64 R = L;
        for(i64 l = 18; l--; ) if(K & 1 << l) {
            auto p = nx[l].lower_bound(R);
            if(p == nx[l].end()) {
                if(ans == INF) ans = -1;
                cout << ans << endl;
                return 0;
            }
            R = p->second;
        }
        chmin(ans, R - L);
    }
    cout << ans << endl;
}