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