#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define int ll
const int N = 2e5 + 7;
const int inf = 1e18;
int n, m, f[N][2], a[N], b[N], res = -inf;
signed main()
{
    ios::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    cin >> n >> m;
    for (int i = 1; i <= n; i++)
        cin >> a[i] >> b[i];
    memset(f, 0x80, sizeof(f));
    f[1][0] = max({b[1], a[1] * m, a[1] * (m - 1) + b[1]});
    for (int i = 2; i <= n; i++)
    {
        f[i][0] = f[i - 1][0] + max({b[i], a[i] * m, a[i] * (m - 1) + b[i]});
        f[i][1] = max(f[i - 1][0], f[i - 1][1]) + max(a[i], b[i]);
    }
    for (int i = 1; i <= n; i++)
        res = max({res, f[i][0], f[i][1]});
    cout << res;
}