#include <bits/stdc++.h>
using namespace std;
using ll = long long;
 
int main(){
    ll d, k, a, b, t;
    scanf("%lld %lld %lld %lld %lld", &d, &k, &a, &b, &t);
 
    // 1) Xác định n (tương đương m = n/k)
    ll m;
    if (t >= k * (b - a)) {
        // Δ ≥ 0 ⇒ nhỏ nhất chọn m = 1 ⇒ n = k
        m = 1;
    } else {
        // Δ < 0 ⇒ chọn m lớn nhất: m = floor(d/k)
        m = d / k;
    }
 
    // 2) Tính Δ và C0
    ll Delta = k * a + t - k * b;   // = k*a + t - k*b
    ll C0    = d * b - t;           // = d*b - t
 
    // 3) Tính f(n) = m*Δ + C0
    ll answer = m * Delta + C0;
    printf("%lld\n", answer);
    return 0;
}
 

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