#include <bits/stdc++.h>
using namespace std;
int main() {
  int c, v, vv, a, l;
  cin >> c >> v >> vv >> a >> l;
  int cnt = 0;
  int s = 0;
  while (true) {
    cnt += 1;
    s = s + v;
    v += a;
    if (v > vv) {
      v = vv;
    }
    if (s >= c) {
      cout << cnt << endl;
      break;
    }
    s -= l;
  }
}
