#include<iostream>
#include<cmath>
#include<cstdio>
#include<cassert>
#define int __int128
using namespace std;
int qpow(int x,int y)
{
	if(y == 0) return 1;
	int k = qpow(x,y / 2);
	if(y & 1) return k * k * x;
	else return k * k;
}
inline int read()
{
	int x = 0, f = 1;
	char ch = getchar();
	while(!isdigit(ch))
	{
		if(ch == '-') f = -1;
		ch = getchar();
	}
	while(isdigit(ch))
	{
		x = (x << 3) + (x << 1) + (ch ^ 48);
		ch = getchar();
	}
	return x * f;
}
void write(__int128 x)
{
	if(x < 10) putchar('0' + x);
	else write(x / 10), putchar('0' + x % 10);
}
signed main()
{
	long long n,x,y;
	__int128 ans = 9e18 + 1 + 1;
//	n = read(); x = read(); y = read();
	cin >>  x >> y >> n;
	int up = log(n) / log(2) + 1;
//	cout << up << endl;
	for(int i = 1; i <= up; i++)
	{
//		assert(pow(n,1.0 / i)>=0);
		int u = floor(pow(n,1.0 / i)) + 1, cnt = 0;
		__int128 xm = 1;
		for(int j = 1; j <= i; j++) xm *= u;
//		assert(xm>=0);
		while(xm >= n)
		{
			xm /= u;
			xm *= (u - 1);
			cnt++;
		}
		xm /= (u - 1);
		xm *= u;
//		assert(xm>=0);
		cnt--;
		ans = min(ans, (__int128)i * x + (__int128)cnt * (u - 2) * y + (__int128)(i - cnt) * (u - 1) * y);
//		assert(ans>=0);
	}
	write(ans);
cout << endl;
	return 0;
}