#include<iostream>
#include<cstdint>
using namespace std; // 575
const int64_t INF2 = 9223372036854775807; // 2^50-1、使わない辺とかに用いる
int main() {
	int64_t N, M, A, B;
	cin >> N >> M >> A >> B;
	int64_t dp[3] = {max(max(M * A, (M - 1) * A + B), B), -INF2, -INF2};
	while (--N) {
		cin >> A >> B;
		dp[2] = max(max(dp[0], dp[1]), dp[2]);
		dp[1] = max(dp[0], dp[1]) + max(A, B);
		dp[0] = dp[0] + max(max(M * A, (M - 1) * A + B), B);
	}
	cout << max(max(dp[0], dp[1]), dp[2]) << endl;
	return 0;
}