#include<bits/stdc++.h>
#define int __int128
using namespace std;
int read()
{
	int ans=0,fh=1;
	char ch=getchar();
	while(ch>'9'||ch<'0')
	{
		if(ch=='-') fh=-1;
		ch=getchar();
	}
	while(ch>='0'&&ch<='9')
	{
		ans=ans*10+ch-'0';
		ch=getchar();
	}
	return ans*fh;
}
const int INF=1e18;
int n;
int logg(int n)
{
	int cnt=0;
	int tmp=1;
	while(tmp<=n)
	{
		tmp*=2;
		cnt++;
	}
	return cnt;
}

int xs(int x,int y)
{
	if(x%y) return x/y+1;
	else return x/y;
}
int ksm(int x,int k)
{
	if(k==0) return 1;
	int res=ksm(x,k/2);
	if(res>=n) return n;
	if(k%2) return res*res*x;
	return res*res;
}
int erfen(int x)
{
	int l=1,r=n;
	while(l<=r)
	{
		int mid=(l+r)>>1;
		if(ksm(mid,x)>=n) r=mid-1;
		else l=mid+1;
	}
	int ans=l*x;
	for(int i=1;i<=x;i++)
	{
		if(ksm(l,x-i)*ksm(l-1,i)>=n) ans--;
		else break;
	}
	return ans-x;
}
void print(int n)
{
	if(n>=10) print(n/10);
	putchar(n%10+'0');
	return;
}
signed main()
{
//	freopen("dice.in","r",stdin);
//	freopen("dice.out","w",stdout);
	int x,y;
	x=read();y=read();n=read();
	int lg=logg(n);
	int ans=INF;
	for(int i=1;i<=lg;i++)
	{
		ans=min(ans,erfen(i)*y+i*x);
	}
	print(ans);
	return 0;
}