#include <algorithm>
#include <iostream>
#include <iomanip>
#include <cstring>
#include <string>
#include <vector>
#include <random>
#include <bitset>
#include <queue>
#include <cmath>
#include <stack>
#include <set>
#include <map>
typedef long long ll;
using namespace std;
const ll MOD = 1000000007LL;

int main() {
    cin.sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);

    ll n, m;
    cin >> n >> m;

    ll a[n], b[n];
    for (int i = 0; i < n; i++) {
        cin >> a[i] >> b[i];
    }

    vector <ll> sm(n+1, 0);
    ll mx = 0;
    for (int i = 0; i < n; i++) {
        sm[i+1] = a[i] + sm[i];
        mx = max(mx, sm[i+1]);
    }

    ll ans = mx * (m - 1);

    vector <ll> opt(n+1, 0);
    mx = 0;
    for (int i = 0; i < n; i++) {
        sm[i+1] = max(a[i], b[i]) + sm[i];
        mx = max(mx, sm[i+1]);
    }

    cout << ans + mx << "\n";
    return 0;
}