#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(false);
  int c, v0, v1, a, l;
  cin >> c >> v0 >> v1 >> a >> l;
  int sum = 0;
  int cur = v0;
  int page_complete = 0;
  for (int i = 1;; i++) {
    page_complete = page_complete + cur;
    if (page_complete >= c) {
      cout << i;
      return 0;
    }
    cur = cur + a;
    if (cur >= v1) {
      cur = v1;
    }
    page_complete -= l;
  }
  cout << sum;
  return 0;
}
