#include <bits/stdc++.h>
using namespace std;
long long ta[1000000], la[1000000];
int main() {
  long long n, L, a, ti, li, i, dif, num = 0;
  cin >> n >> L >> a;
  for (i = 0; i < n; i++) {
    cin >> ta[i];
    cin >> la[i];
    la[i] += ta[i];
  }
  if (n == 0) {
    num = L / a;
    cout << num;
    return 0;
  }
  dif = (ta[0] - 0) / a;
  num += dif;
  for (i = 1; i < n; i++) {
    dif = (ta[i] - la[i - 1]) / a;
    num += dif;
  }
  dif = (L - la[n - 1]) / a;
  num += dif;
  cout << num;
  return 0;
}
