#include <bits/stdc++.h>   using namespace std;   const int maxn = 1000 + 10;   int main() {     int c, d, n, m, k;     while (scanf( %d %d , &c, &d) != EOF) {       scanf( %d %d , &n, &m);       scanf( %d , &k);       int t = n * m - k;       if (t <= 0) {         printf( 0 n );         continue;       }       int res = t / n;       int ans = min(res * n * d, res * c);       ans += min(c, (t - res * n) * d);       cout << ans << endl;     }     return 0;   }   