#include<bits/stdc++.h>
namespace {
#pragma GCC diagnostic ignored "-Wunused-function"
#include<atcoder/all>
#pragma GCC diagnostic warning "-Wunused-function"
using namespace std;
using namespace atcoder;
#define rep(i,n) for(int i = 0; i < (int)(n); i++)
#define rrep(i,n) for(int i = (int)(n) - 1; i >= 0; i--)
#define all(x) begin(x), end(x)
#define rall(x) rbegin(x), rend(x)
template<class T> bool chmax(T& a, const T& b) { if (a < b) { a = b; return true; } else return false; }
template<class T> bool chmin(T& a, const T& b) { if (b < a) { a = b; return true; } else return false; }
using ll = long long;
using P = pair<int,int>;
using VI = vector<int>;
using VVI = vector<VI>;
using VL = vector<ll>;
using VVL = vector<VL>;

} int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int n, d;
  cin >> n >> d;
  vector<P> ab(n);
  for (auto& [a, b] : ab) cin >> b >> a;
  sort(rall(ab));
  ll base = 0;
  int ptr = 0;
  {
    ll acc = 0;
    while (ptr < n && acc + ab[ptr].second <= d) acc += ab[ptr++].second;
  }
  int v = ptr == n ? 0 : ab[ptr].first;
  for (auto& [a, b] : ab | views::take(ptr)) {
    // a-dk >= v
    // dk <= a-v
    int k = (a - v) / d;
    base += (ll)k * b;
    a -= (ll)k * d;
  }
  sort(rall(ab));
  ll ans = base + ab[0].first;
  ll acc = 0;
  rep(i, n + 1) {
    chmin(ans, base + acc + (i == n ? 0 : ab[i].first));
    if (i < n) {
      auto [a, b] = ab[i];
      acc += b;
    }
  }
  cout << ans << '\n';
}