#include <bits/stdc++.h>
const int Max = 1e6;
using namespace std;
long long a, b, w, x, c, sum, sum1;
int main() {
  scanf("%I64d%I64d%I64d%I64d%I64d", &a, &b, &w, &x, &c);
  if (c <= a)
    printf("0\n");
  else {
    sum = ceil((1.0 * (c - a) * x - b) / (w - x));
    printf("%I64d\n", sum + c - a);
  }
  return 0;
}
