#include <bits/stdc++.h>
int main() {
  int n, v, pre, rotting, a[4005], b[4005], i, j, bag, collect, day, v2;
  while (scanf("%d%d", &n, &v) != EOF) {
    day = 0;
    bag = 0;
    for (i = 0; i < n; i++) {
      scanf("%d%d", &a[i], &b[i]);
      if (a[i] > day) day = a[i];
    }
    rotting = 0;
    day += 2;
    for (i = 1; i < day; i++) {
      pre = 0;
      for (j = 0; j < n; j++) {
        if (a[j] == i) {
          pre += b[j];
        }
      }
      if (rotting > v) {
        bag += v;
        v2 = 0;
      } else {
        bag += rotting;
        v2 = v - rotting;
      }
      if (pre > v2) {
        bag += v2;
        pre -= v2;
      } else {
        bag += pre;
        pre = 0;
      }
      rotting = pre;
    }
    printf("%d\n", bag);
  }
  return 0;
}
