#include <iostream>
#include <algorithm>
#include <cstring>
#include <vector>
#include <queue>
#include <unordered_map>
#include <unordered_set>
#include <map>
#include <set>
#include <cmath>
#include <bitset>
#include <cassert>
#include <numeric>

#ifdef LOCAL
#include "../debug/debug.hpp"
#else
#define debug(...) 233
#endif

using namespace std;
typedef long long ll;

void solve()
{
    int n, m;
    cin >> n >> m;
    vector<int> a(n + 1), b(n + 1);
    for (int i = 1; i <= n; i ++)
        cin >> a[i] >> b[i];
    
    vector<ll> s(n + 1), suf(n + 1);
    for (int i = 1; i <= n; i ++)
        s[i] = s[i - 1] + max(a[i], b[i]);
    
    suf[n] = s[n];
    for (int i = n - 1; i >= 1; i --)
        suf[i] = max(suf[i + 1], s[i]);
    
    ll cost = 0;
    ll res = 0;
    ll ans = 0;
    for (int i = 1; i <= n; i ++)
    {
        if (a[i] >= 0)
        {
            res += 1ll * (m - 1) * a[i];
        }
        else if (a[i] < 0 && b[i] < 0)
        {
            res += max(1ll * m * a[i], 1ll * b[i]);
            cost += max(a[i], b[i]);
        }
        ans = max(ans, res + suf[i] - cost);
    }

    cout << ans << '\n';
}

int main()
{
    // freopen("remake.in", "r", stdin);
    // freopen("remake.out", "w", stdout);
    
    ios::sync_with_stdio(0), cin.tie(0), cout.tie(0);

    int tt = 1;
    // cin >> tt;
    while (tt --)
    {
        solve();
    }
    return 0;
}