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