#include <bits/stdc++.h>
using namespace std;
#define int long long
#define fi first
#define se second
#define rep(i, s, n) for (int i = s; i < n; i++)
#define rrep(i, s, n) for (int i = (n)-1; i >= (s); i--)
const long long MOD = 1e9 + 7, INF = 1e18;

template <class T>
inline bool chmax(T &a, T b)
{
    if (a < b)
    {
        a = b;
        return true;
    }
    return false;
}

signed main()
{
    int N, M;
    cin >> N >> M;
    vector<int> a(N), b(N);
    int last = 0, best = 0, lastmax = 0, bestmax = 0;
    for (int i = 0; i < N; i++)
    {
        cin >> a[i] >> b[i];
        last += max(a[i], b[i]);
        best += a[i];
        chmax(lastmax, last);
        chmax(bestmax, best);
    }
    cout << bestmax * (M - 1) + lastmax << endl;
}