#include <bits/stdc++.h>
using namespace std;
void doRoutine() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
}
ifstream fin("/home/max/user/alg/codeforces_clion/input.txt");
ofstream fout("/home/max/user/alg/codeforces_clion/output.txt");
signed main() {
  doRoutine();
  int64_t c, v0, v1, a, l;
  cin >> c >> v0 >> v1 >> a >> l;
  int64_t ans = 0, cur_pages = 0, cur_v = v0;
  bool is_first_day = true;
  while (true) {
    ++ans;
    int64_t last = cur_pages + cur_v;
    if (!is_first_day) {
      last -= l;
    } else {
      is_first_day = false;
    }
    if (last >= c) {
      cout << ans << endl;
      return 0;
    }
    cur_pages = last;
    cur_v = min(cur_v + a, v1);
  }
  return 0;
}
