#include <bits/stdc++.h>
using namespace std;
int inf_int = 2e9;
long long inf_ll = 2e18;
const double pi = 3.1415926535898;
template <typename T, typename T1>
void prin(vector<pair<T, T1> >& a) {
  for (int i = 0; i < a.size(); i++) {
    cout << a[i].first << " " << a[i].second << "\n";
  }
}
template <typename T, typename T1>
void prin(set<pair<T, T1> >& a) {
  for (auto it = a.begin(); it != a.end(); it++) {
    cout << it->first << " " << it->second << "\n";
  }
}
template <typename T>
void prin(vector<T>& a) {
  for (int i = 0; i < a.size(); i++) {
    cout << a[i];
    if (i < a.size() - 1)
      cout << " ";
    else
      cout << "\n";
  }
}
template <typename T>
void prin(set<T>& a) {
  for (auto it = a.begin(); it != a.end(); it++) {
    cout << *it << " ";
  }
}
template <typename T>
void prin_new_line(vector<T>& a) {
  for (int i = 0; i < a.size(); i++) {
    cout << a[i] << "\n";
  }
}
template <typename T, typename T1>
void prin_new_line(vector<pair<T, T1> >& a) {
  for (int i = 0; i < a.size(); i++) {
    cout << a[i].first << " " << a[i].second << "\n";
  }
}
int sum_vec(vector<int>& a) {
  int s = 0;
  for (int i = 0; i < a.size(); i++) {
    s += a[i];
  }
  return s;
}
template <typename T>
T max(vector<T>& a) {
  T ans = a[0];
  for (int i = 1; i < a.size(); i++) {
    ans = max(ans, a[i]);
  }
  return ans;
}
template <typename T>
T min(vector<T>& a) {
  T ans = a[0];
  for (int i = 1; i < a.size(); i++) {
    ans = min(ans, a[i]);
  }
  return ans;
}
template <typename T>
T min(T a, T b, T c) {
  return min(a, min(b, c));
}
template <typename T>
T max(T a, T b, T c) {
  return max(a, max(b, c));
}
long double s_triangle(long double x1, long double y1, long double x2,
                       long double y2, long double x3, long double y3) {
  return abs(((x2 - x1) * (y3 - y1) - (x3 - x1) * (y2 - y1)) / 2);
}
bool overflow(long long a, long long b) {
  if (a * b / b != a) return true;
  return false;
}
bool debug = 0;
const int maxn = 3e5 + 100;
long long mod = 1e9 + 7;
int t[4 * maxn];
void build(int v, int tl, int tr) {
  if (tl == tr) {
    t[v] = 1;
  } else {
    int tm = (tl + tr) >> 1;
    build(v << 1, tl, tm);
    build(v << 1 | 1, tm + 1, tr);
    t[v] = t[v << 1] + t[v << 1 | 1];
  }
}
int get(int v, int tl, int tr, int l, int r) {
  if (l > r) return 0;
  if (tl == l && tr == r) {
    return t[v];
  } else {
    int tm = (tl + tr) >> 1;
    return get(v << 1, tl, tm, l, min(r, tm)) +
           get(v << 1 | 1, tm + 1, tr, max(tm + 1, l), r);
  }
}
int ans;
void find_and_delete(int v, int tl, int tr, int val) {
  if (tl == tr) {
    ans = tl;
    t[v] = 0;
  } else {
    int tm = (tl + tr) >> 1;
    int gfgs = t[v << 1];
    int x2 = t[v << 1 | 1];
    if (gfgs >= val) {
      find_and_delete(v << 1, tl, tm, val);
    } else {
      find_and_delete(v << 1 | 1, tm + 1, tr, val - gfgs);
    }
    t[v] = t[v << 1] + t[v << 1 | 1];
  }
}
void solve() {
  long long d, k, a, b, t;
  cin >> d >> k >> a >> b >> t;
  long long ans = 0;
  if (d <= k) {
    cout << a * d;
    return;
  }
  ans += a * k;
  d = d - k;
  double v1 = (1.0 * a * k + t) / k;
  if (v1 < b) {
    long long x = d / k;
    ans += x * (a * k + t);
    d = d - x * k;
  } else {
    ans += d * b;
    cout << ans;
    return;
  }
  ans += min(b * d, a * d + t);
  cout << ans;
}
int main() {
  if (!debug) {
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
  }
  int t = 1;
  while (t--) solve();
  return 0;
}
