#include <bits/stdc++.h>
using namespace std;
signed main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  int c, v0, v1, a, l;
  cin >> c >> v0 >> v1 >> a >> l;
  for (int day = 0; true; day++) {
    int read = min(v1, v0 + day * a);
    if (day > 0) c += l;
    c -= read;
    if (c <= 0) {
      cout << day + 1 << endl;
      break;
    }
  }
}
