#include <bits/stdc++.h>

using namespace std;

using ll = long long;

const ll INF = 1e15;

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    int n, m;
    cin >> n >> m;
    vector<ll> a(n), b(n);
    for (int i = 0; i < n; i++) {
        cin >> a[i] >> b[i];
    }

    int idx1 = -1;
    ll maxv1 = -INF;
    ll sum1 = 0;
    for (int i = 0; i < n; i++) {
        sum1 += max(a[i], b[i]);
        if (sum1 >= maxv1) {
            idx1 = i;
            maxv1 = sum1;
        }
    }

    vector<ll> memo(idx1 + 1, 0);
    for (int i = idx1 - 1; i >= 0; i--) {
        memo[i] = memo[i + 1] + max(a[i + 1], b[i + 1]);;
    }

    ll sum2 = 0;
    ll ans = -INF;
    for (int i = 0; i <= idx1; i++) {
        sum2 += max({a[i] * (m - 1) + b[i], a[i] * m, b[i]});
        ans = max(ans, sum2 + memo[i]);
    }

    cout << ans << endl;
    return 0;
}