#include <bits/stdc++.h>
using namespace std;
int main() {
  long long c, v0, v1, a, l, c1 = 0, c2 = 0;
  cin >> c >> v0 >> v1 >> a >> l;
  while (true) {
    c1++;
    c2 += v0;
    if (c2 >= c) {
      cout << c1;
      return 0;
    }
    v0 += a;
    v0 = min(v0, v1);
    c2 -= l;
    c = max(c, (long long)0);
  }
  return 0;
}
