#include <bits/stdc++.h>
using namespace std;
const int maxn = 1e5 + 5;
int n, L, a;
int t[maxn], l[maxn];
int main() {
  scanf("%d%d%d", &n, &L, &a);
  for (int i = 0; i < n; i++) {
    scanf("%d%d", &t[i], &l[i]);
  }
  int start = 0;
  int ans = 0;
  for (int i = 0; i < n; i++) {
    ans += (t[i] - start) / a;
    start = t[i] + l[i];
  }
  ans += (L - start) / a;
  printf("%d", ans);
  return 0;
}
