#include <bits/stdc++.h>
int main(void) {
  int c, vmin, vmax, a, re;
  int v, day;
  int i, j, k;
  scanf("%d%d%d%d%d", &c, &vmin, &vmax, &a, &re);
  v = vmin;
  day = 1;
  while (c > v) {
    if (c <= v) break;
    c -= v;
    v += a;
    if (v > vmax) v = vmax;
    c += re;
    day++;
  }
  printf("%d\n", day);
  return 0;
}
