#include <bits/stdc++.h>
using namespace std;
void fun() {}
signed main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  ;
  fun();
  long long int c, v, u, a, l, p = 0, ans = 1;
  cin >> c >> v >> u >> a >> l;
  p += v;
  while (p < c) {
    v += a;
    p += min(v, u) - l;
    ans++;
  }
  cout << ans;
  cerr << "time elapsed : " << 1.0 * clock() / CLOCKS_PER_SEC << " sec \n";
  return 0;
}
