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