#include <bits/stdc++.h>
using namespace std;
int main() {
  int c, v0, v1, a, l;
  cin >> c >> v0 >> v1 >> a >> l;
  int d = 1;
  int r = v0;
  int tmp = v0;
  while (1) {
    if (r >= c) break;
    tmp = tmp + a;
    if (tmp > v1) tmp = v1;
    r += (tmp - l);
    d++;
  }
  cout << d << endl;
  return 0;
}
