#include <bits/stdc++.h>
using namespace std;
int main() {
  long long int c, vs, vm, a, l, d = 1, cur;
  cin >> c;
  cin >> vs;
  cin >> vm;
  cin >> a;
  cin >> l;
  cur = vs;
  while (cur < c) {
    cur = cur - l;
    if ((vs + d * a) < vm)
      cur += vs + d * a;
    else
      cur += vm;
    d++;
  }
  cout << d;
}
