#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  int n, v, a, b, x;
  cin >> n >> v;
  int mp[3002] = {0};
  for (int i = 0; i < n; i++) {
    cin >> a >> b;
    mp[a] += b;
  }
  int prev = 0, ans = 0;
  for (int i = 1; i <= 3001; i++) {
    int now = mp[i];
    if (now + prev <= v) {
      ans += (now + prev);
      prev = 0;
    } else {
      ans += v;
      prev = now - max(v - prev, 0);
    }
  }
  cout << ans << '\n';
  return 0;
}
