#include <bits/stdc++.h>
#include <atcoder/segtree>
using namespace atcoder;
using namespace std;
using ll = long long;
int N;
ll M;
int MN = 2e6;
vector<ll> A(MN), B(MN);
ll INF = (1LL << 60);
ll score(int i, int j) {
	if (j == 0) return 0LL;
	ll d = j;
	if (d == 2) d = M;
	return max(d * A[i], max(((d - 1) * A[i]) + B[i], B[i]));
}
int main() {
	cin >> N >> M;
	for (int i = 1;i <= N;i++) cin >> A[i] >> B[i];
	vector<vector<ll>> dp(N + 1, vector<ll>(3, -INF));
	dp[1][2] = score(1, 2);
	for (int i = 2;i <= N;i++) for (int j = 0;j < 3;j++) for (int k = j;k < 3;k++) {
		if (dp[i - 1][k] == -INF) continue;
		dp[i][j] = max(dp[i][j], dp[i - 1][k] + score(i, j));
	}
	cout << max(dp[N][0], max(dp[N][1], dp[N][2])) << endl;
}