#include <bits/stdc++.h>
using namespace std;
int main() {
  int c, v, v1, a, l;
  cin >> c >> v >> v1 >> a >> l;
  int pages_read = 0;
  int days = 0;
  while (1) {
    pages_read += (min(v + (days * a), v1));
    days++;
    if (pages_read >= c) break;
    pages_read -= l;
  }
  cout << days << endl;
}
