#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr);
  int c, l, v0, v1, a;
  cin >> c >> v0 >> v1 >> a >> l;
  int day, read = v0, v = v0;
  for (day = 1; read < c; ++day) {
    read -= l;
    v = min(v + a, v1);
    read += v;
  }
  cout << day;
}
