#include <bits/stdc++.h>
using namespace std;
template <class T>
inline void read(T &x) {
  x = 0;
  int f = 1;
  char ch = getchar();
  while (!isdigit(ch)) {
    if (ch == '-') f = -1;
    ch = getchar();
  }
  while (isdigit(ch)) {
    x = x * 10 + ch - 48;
    ch = getchar();
  }
  x *= f;
}
int n, L, a;
int ans;
int main() {
  read(n), read(L), read(a);
  int s = 0, x, y;
  for (register int i = 1; i <= n; ++i)
    read(x), read(y), ans += max(0, (x - s) / a), s = x + y;
  ans += max(0, (L - s) / a);
  printf("%d\n", ans);
}
