#include<bits/stdc++.h>
using namespace std;
int main() {
	int N, K;
	cin >> N >> K;
	std::vector<pair<int, int>> RL(N) ;
	for (auto& [r, l] : RL) {
		cin >> l >> r;
	}
	sort(RL.begin(), RL.end());
	vector<int> nxt[20];
	nxt[0].resize(N + 1); nxt[0][N] = N;
	{
		int fl = 0;
		for (int i = 0; i < N; i ++) {
			while (fl < N && RL[fl].second < RL[i].first) fl ++;
			nxt[0][i] = fl;
		}
	}
	for (int i = 1; i < 20; i ++) {
		for (int j = 0; j < N; j ++) {
			nxt[i][j] = nxt[i - 1][nxt[i - 1][j]];
		}
	}
	int ans = -1;
	K --;
	for (int i = 0; i < N; i ++) {
		int j = i;
		for (int i = 0; i < 20; i ++) {
			if ((K >> i) & 1) {
				j = nxt[i][j];
			}
		}
		if (j < N) {
			int d = RL[j].first - RL[i].second;
			if (ans == -1) ans = d;
			else ans = min(ans, d);
		}
	}
	cout << ans << endl;
}