#include <bits/stdc++.h>
using namespace std;
const int MAXN = 100010;
int S[MAXN], T[MAXN];
int main() {
  int n, L, a, ans = 0;
  T[0] = 0;
  S[0] = 0;
  S[1] = 0;
  scanf("%d%d%d", &n, &L, &a);
  for (int i = 1; i <= n; i++) scanf("%d%d", &S[i], &T[i]);
  for (int i = 1; i < n; i++) {
    ans += (S[i + 1] - S[i] - T[i]) / a;
  }
  ans += (L - S[n] - T[n]) / a;
  ans += S[1] / a;
  printf("%d\n", ans);
  return 0;
}
