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