#include <bits/stdc++.h>
using namespace std;
int main() {
  int c, v, v1, a, l, ans = 0;
  cin >> c >> v >> v1 >> a >> l;
  int ok = 0;
  while (ok < c) {
    ++ans;
    ok = max(ok - l, 0);
    ok += min(v, v1);
    v += a;
  }
  cout << ans;
  exit(0);
}
