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