#include <bits/stdc++.h>
using namespace std;
int main()
{
    int A, B, N;
    cin >> A >> B >> N;

    if(N == 1)
    {
        cout << 0 << endl;
        return 0;
    }

    long long ans = LLONG_MAX;
    for (int i = 1; i <= 30; ++i)
    {
        int ret = pow(N, 1. / i);
        for (int j = 0; j <= i; ++j)
        {
            long long imm = 0, num = 1;
            for (int k = 0; k < i; ++k)
            {
                if (k < j)
                {
                    imm += 1LL * ret * B + A;
                    num *= ret + 1;
                }
                else
                {
                    imm += 1LL * (ret - 1) * B + A;
                    num *= ret;
                }
                if (num >= N)
                    break;
            }

            if (num >= N)
                ans = min(ans, imm);
        }
    }
    cout << ans << endl;
}