#include <bits/stdc++.h>
int main() {
  long int i, j, n, m, l, a, tt1, ll1, tt2, ll2, time, gap, ans = 0;
  scanf("%ld%ld%ld", &n, &l, &a);
  time = l;
  ans = 0;
  tt1 = 0;
  ll1 = 0;
  tt2 = 0;
  ll2 = 0;
  for (i = 1; i <= n; i++) {
    scanf("%ld%ld", &tt2, &ll2);
    gap = tt2 - tt1 - ll1;
    ans = ans + gap / a;
    tt1 = tt2;
    ll1 = ll2;
  }
  time = time - tt2 - ll2;
  ans = ans + (time / a);
  printf("%ld\n", ans);
  return 0;
}
