#include <bits/stdc++.h>
using namespace std;
const int MAXN = (int)1e5 + 10;
const long long MOD = (long long)1e9 + 7;
int main() {
  int c, v1, v0, a, l;
  cin >> c >> v0 >> v1 >> a >> l;
  int ans = 0, tol = 0;
  while (tol < c) {
    ans++;
    tol += v0;
    if (ans > 1) tol -= l;
    v0 += a;
    v0 = min(v1, v0);
  }
  cout << ans << endl;
  return 0;
}
