#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;
typedef long long ll;

const ll INF = 1e18; // ????????

int main() {
    // ???????
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);

    int n;
    ll m;
    if (!(cin >> n >> m)) return 0;

    vector<ll> a(n + 1), b(n + 1);
    for (int i = 1; i <= n; i++) {
        cin >> a[i] >> b[i];
    }

    // pref_P[i] ?? max(A[j], B[j]) ???? (1 <= j <= i)
    vector<ll> pref_P(n + 1, 0);
    // pref_A[i] ?? A[j] ???? (1 <= j <= i)
    vector<ll> pref_A(n + 1, 0);
    
    // ??????
    for (int i = 1; i <= n; i++) {
        pref_P[i] = pref_P[i - 1] + max(a[i], b[i]);
        pref_A[i] = pref_A[i - 1] + a[i];
    }

    // max_pref_A[i] ??? 1 <= Y <= i ??????? pref_A[Y]
    vector<ll> max_pref_A(n + 1, -INF);
    max_pref_A[1] = pref_A[1]; // ???? K >= 1????????????
    for (int i = 2; i <= n; i++) {
        max_pref_A[i] = max(max_pref_A[i - 1], pref_A[i]);
    }

    ll ans = -INF;

    // ?? M ??????? K ??????? X?
    for (int X = 1; X <= n; X++) {
        // ????????1 ? X ? X ?????? max(A, B)
        ll base_score = pref_P[X];
        
        // ??????????? M-1 ????????????? Y (1 <= Y <= X)
        // ?? A ??????
        ll extra_score = 0;
        if (m > 1) {
            extra_score = (m - 1) * max_pref_A[X];
        }
        
        ans = max(ans, base_score + extra_score);
    }

    cout << ans << "\n";

    return 0;
}