#include <vector>
#include <iostream>
#include <algorithm>
using namespace std;
int main() {
	int N, D;
	cin >> N >> D;
	vector<int> P(N), Q(N);
	for (int i = 0; i < N; ++i) {
		cin >> P[i] >> Q[i];
	}
	int answer = -(1 << 30);
	for (int i = 0; i < N; ++i) {
		for (int j = 0; j < N; ++j) {
			if (i == j) continue;
			int ans = -P[i];
			ans = min(ans, -P[i] + Q[i] - P[j]);
			ans = min(ans, (Q[i] - P[i]) * ((D - 1) / 2) + (Q[j] - P[j]) * ((D - 2) / 2) - (D % 2 == 0 ? P[i] : P[j]));
			ans = min(ans, (Q[i] - P[i]) * (D / 2) + (Q[j] - P[j]) * ((D - 1) / 2) - (D % 2 == 0 ? P[j] : P[i]));
			answer = max(answer, ans);
		}
	}
	cout << answer << endl;
	return 0;
}