#include <bits/stdc++.h>
using namespace std;
long long t[100100], l[100100];
int main() {
  long long n, k, ln, a, p = 0;
  cin >> n >> ln >> a;
  t[0] = 0;
  l[0] = 0;
  for (k = 1; k <= n; k++) cin >> t[k] >> l[k];
  t[n + 1] = ln;
  for (k = 1; k <= n + 1; k++) p = p + abs(t[k] - (t[k - 1] + l[k - 1])) / a;
  cout << p;
  return 0;
}
