#include <bits/stdc++.h>

using namespace std;

const int maxN = 1e5 + 5;
const long long oo = 1e16;

int n, m;
long long a[maxN], b[maxN];
long long dp[maxN][2];

signed main() {
    freopen("test.inp", "r", stdin);
    freopen("test.out", "w", stdout);
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);cout.tie(nullptr);
    cin >> n >> m;
    for (int i = 1; i <= n; i++) {
        cin >> a[i] >> b[i];
    }

    dp[0][0] = 0;
    dp[0][1] = -oo;
    long long ans = 0;
    for (int i = 1; i <= m; i++) {
        dp[i][0] = dp[i - 1][0] + max({a[i] * m, b[i], a[i] * (m - 1) + b[i]});
        dp[i][1] = max(dp[i - 1][0], dp[i - 1][1]) + max(a[i], b[i]);
        ans = max({ans, dp[i][0], dp[i][1]});
    }
    cout << ans << '\n';
}