#include <bits/stdc++.h>
using namespace std;
int main() {
  int c, v0, l, v1, a;
  cin >> c >> v0 >> v1 >> a >> l;
  int i = 0;
  int cc = 0;
  while (cc < c) {
    cc += (min(v1, v0 + i * a));
    if (i) cc -= l;
    i++;
  }
  cout << (i) << endl;
}
