#define _USE_MATH_DEFINES
#include "bits/stdc++.h"
using namespace std;
#define FOR(i,j,k) for(int (i)=(j);(i)<(int)(k);++(i))
#define rep(i,j) FOR(i,0,j)
#define each(x,y) for(auto &(x):(y))
#define mp make_pair
#define MT make_tuple
#define all(x) (x).begin(),(x).end()
#define debug(x) cout<<#x<<": "<<(x)<<endl
#define smax(x,y) (x)=max((x),(y))
#define smin(x,y) (x)=min((x),(y))
#define MEM(x,y) memset((x),(y),sizeof (x))
#define sz(x) (int)(x).size()
#define RT return
using ll = long long;
using pii = pair<int, int>;
using vi = vector<int>;
using vll = vector<ll>;

const ll MI = LLONG_MIN / 3;
ll dp[100005][3];

void solve() {
    int N;
    ll M;
    cin >> N >> M;

    rep(i, N + 1)rep(j, 3)dp[i][j] = MI;
    dp[0][2] = 0;

    rep(i, N) {
        // 卵、肉
        int A, B;
        cin >> A >> B;

        rep(j, 3) {
            rep(jj, j + 1) {
                if (jj == 0) {
                    smax(dp[i + 1][0], dp[i][j]);
                } else if (jj == 1) {
                    smax(dp[i + 1][1], dp[i][j] + max(A, B));
                } else {
                    ll va = max<ll>(max<ll>(A*M, B), A*(M - 1) + B);
                    smax(dp[i + 1][2], dp[i][j] + va);
                }
            }
        }
    }

    ll ans = MI;
    rep(i, 3)smax(ans, dp[N][i]);
    cout << ans << endl;
}

int main() {
	ios::sync_with_stdio(false);
	cin.tie(0);
	cout << fixed << setprecision(15);
	solve();
	return 0;
}