#include <bits/stdc++.h>

using namespace std;

using ll = long long;

int main() {
  int N;
  ll D;
  cin >> N >> D;
  vector<pair<ll, ll> > V(N);
  ll total = 0;
  for (auto &[b, a]: V) {
    cin >> a >> b;
    total += a;
  }
  ranges::sort(V, greater());
  ll K = min(D, total);
  ll base = 0, cur = 0;
  map< ll, ll > events;
  for (auto [b, a]: V) {
    auto take = min(a, K);
    if (take == 0) break;
    base += take * (b / D);
    if (b % D != 0) {
      events[b % D] += take;
      cur += take;
    }
    K -= take;
  }
  ll add = cur;
  for (auto [r, cnt] : events) {
    cur -= cnt;
    add = min(add, r + cur);
  }
  cout << base + add << endl;
}