#include<bits/stdc++.h>
#define int __int128
using namespace std;
int n,a,b,ans=ULLONG_MAX;
int qread(){
	int w=1,ret;
	char c;
	while((c=getchar())>'9'||c<'0')w=(c=='-')?-1:1;
	ret=c-'0';
	while((c=getchar())>='0'&&c<='9')ret=ret*10+c-'0';
	return w*ret;
}
int fpow(int n,int k){
	int ret=1;
	for(;k;k>>=1){
		if(k&1)ret=ret*n;
		n=n*n;
	}
	return ret;
}
bool check(int x,int y){
	int z=1;
	while(y){
		if(z*x>n)return true;
		z*=x;
		y--;
	}
	return false;
}
signed main(){
	a=qread();
	b=qread();
	n=qread()-1;
	for(int i=1;i<=63;i++){
		int l=1,r=n+1,z=1,res=0;
		while(l<r){
			int mid=(l+r)>>1;
			if(check(mid,i))r=mid;
			else l=mid+1;
		}
		z=fpow(r,i);
		res=i*(a+b*(r-1));
		for(int j=1;j<i;j++){
			if(z/r*(r-1)>n){
				z=z/r*(r-1);
				res-=b;
			}
			else break;
		}
		ans=min(res,ans);
	}
	printf("%lld\n",(long long)ans);
    return 0;
}