#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
typedef long long int ll;
typedef long double ld;
#define FOR(i,l,r) for(ll i=l;i<r;i++)
#define REP(i,n) FOR(i,0,n)
#define RFOR(i,l,r) for(ll i=r-1;i>=l;i--)
#define RREP(i,n) RFOR(i,0,n)
#define ALL(x) x.begin(),x.end()
#define P pair<ll,ll>
#define F first
#define S second
#define BS(A,x) binary_search(ALL(A),x)
#define LB(A,x) (ll)(lower_bound(ALL(A),x)-A.begin())
#define UB(A,x) (ll)(upper_bound(ALL(A),x)-A.begin())
#define COU(A,x) (UB(A,x)-LB(A,x))
template<typename T>using min_priority_queue=priority_queue<T,vector<T>,greater<T>>;
using mint=modint1000000007;
//using mint=modint998244353;
void chmax(ll&a,ll b){a=max(a,b);}
void chmin(ll&a,ll b){a=min(a,b);}
void print(vector<ll>&A){REP(i,A.size()){if(i)cout<<" ";cout<<A[i];}cout<<endl;}
ll dist(ll sx,ll sy,ll tx,ll ty){return (sx-tx)*(sx-tx)+(sy-ty)*(sy-ty);}
vector<vector<ld>>pro(vector<vector<ld>>A,vector<vector<ld>>B){
  ll N=A.size();
  vector<vector<ld>>res(N,vector<ld>(N));
  REP(i,N)REP(j,N)REP(k,N)res[i][j]+=A[i][k]*B[k][j];
  return res;
}
map<ll,ll>memo;
ll A,B;
void f(ll N){
  if(memo[N])return;
  ll d=2;memo[N]=A+(N-1)*B;
  while(d*d<=N){
    if(!memo[d])f(d);
    chmin(memo[N],memo[d]+A+(N-1)/d*B);
    if(!memo[(N+d-1)/d])f((N+d-1)/d);
    chmin(memo[N],memo[(N+d-1)/d]+A+(d-1)*B);
    d++;
  }
}
int main(){
  ll N;cin>>A>>B>>N;
  f(N);cout<<memo[N]<<endl;
  return 0;
}