#include<bits/stdc++.h>
#define il inline 
#define re register
#define int long long
using namespace std;
const int N=1e6+10,INF=LONG_LONG_MAX;
#define gc() getchar()
il int rd(){
	int x=0,f=1;
	char ch=gc();
	while(!isdigit(ch)){if(ch=='-')f=-1;ch=gc();}
	while(isdigit(ch))x=(x<<3)+(x<<1)+(ch^48),ch=gc();
	return x*f;
}
int n,x,y;
int f[61];
void Main(){
	x=rd(),y=rd();n=rd();
	int lim=__lg(n),Ans=x+n*y;
	int p=sqrt(n),q=p;
	if(p*p<n)++q;
	Ans=min(Ans,((p-2)+(q-2))*y+2*(x+y));
	for(int i=3; i<=lim+1; ++i){//????copy
		int s=1ll<<i,ans=i*(x+y),head=0;
		for(int j=1; j<=i; ++j)f[j]=2;
		while(s<n){
			++head;if(head>i)head=1;
			s=s/f[head]*(f[head]+1);
			++f[head];ans+=y;
			if(ans>Ans)break;
		}
//		cout<<i<<endl;
		Ans=min(Ans,ans);
	}
	cout<<Ans;
}
/*
  1000000000000000000 1 1
 */
signed main(){
//	freopen("dice.in","r",stdin);
//	freopen("dice.out","w",stdout);
	int T=1;
	while(T--)Main();
	return 0;
}