#include <stdio.h>
#include <bits/stdc++.h>
#include <atcoder/all>
using namespace atcoder;
using mint = modint998244353;
using namespace std;
#define rep(i,n) for(int i=0;i<(n);i++)
#define Inf 1000000000000000000


int main(){
	
	long long n,a,b;
	cin>>a>>b>>n;
	long long ans = Inf;
	if(n==1)ans = 0;
	
	for(int i=1;i<=30;i++){
		long long t = a * i;
		
		rep(j,i+1){
			long long ok = n+1,ng = 1LL;
			while(ok-ng>1LL){
				long long mid = (ok+ng)/2;
				long long cur = n-1;
				rep(k,j){
					cur /= mid;
				}
				rep(k,i-j){
					cur /= mid-1;
				}
				if(cur==0)ok = mid;
				else ng = mid;
			}
			ans = min(ans,t + ((ok-1) * j + (ok-2) * (i-j)) * b);
		}
		
	}
	cout<<ans<<endl;
	
	return 0;
}