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