#include <bits/stdc++.h>
using namespace std;
int main() {
  int vv[3003] = {0}, vvv[3003] = {0};
  int n, v, m = -1, k;
  cin >> n >> v;
  while (n--) {
    int a, b;
    cin >> a >> b;
    vv[a] += b;
    vvv[a + 1] += b;
    m = max(m, a + 1);
  }
  int ans = 0, mm;
  for (int i = 0; i <= m; i++) {
    mm = v;
    if (vvv[i]) {
      if (vvv[i] < mm) {
        ans += vvv[i];
        mm -= vvv[i];
      } else {
        ans += mm;
        mm = 0;
      }
    }
    if (mm) {
      if (vv[i] < mm) {
        ans += vv[i];
        vvv[i + 1] -= vv[i];
      } else {
        ans += mm;
        vvv[i + 1] -= mm;
      }
    }
  }
  cout << ans << endl;
  return 0;
}
