#include <bits/stdc++.h>
using namespace std;
int main() {
  int c, v0, v1, a, l;
  cin >> c >> v0 >> v1 >> a >> l;
  int now = 0, ans = 0, re = v0;
  while (now < c) {
    ans++;
    now = max(0, now - l);
    now += re;
    re += a;
    re = min(re, v1);
  }
  printf("%d\n", ans);
  return 0;
}
