/*
 *  Copyright (c) 2025 Parag Kishor Shornob
 *   All rights reserved.
 */
#include <bits/stdc++.h>
using namespace std;

typedef long long ll;
typedef vector< string > vi;
typedef pair<int,int> pii;

#define yes "YES\n"
#define no "NO\n"
#define pb push_back
#define pop pop_back
#define fastin ios::sync_with_stdio(false); cin.tie(nullptr); cout.tie(nullptr);
#define input(v) for(auto &i  : v) cin>>i;
#define all(x) (x).begin(),(x).end()

char ee = '\n';
void printVec(vector<string> &a){
    for(auto pp : a) cout<<pp <<" " ;
    cout<<ee;
}
ll power(ll base , ll exp){
    ll result=1;
    while(exp>0){
       if(exp%2==1){
        result *=base;
       }
       base  *=base ;
       exp/=2;

    }
    return result;

}

void  solve(){
    string s ; cin>>s ;
    ll a = s[0]-'0';
    ll b = s[2]-'0';
    ll n ; cin>>n ;
    ll ans =a;
    ll d= b-a;
    ll sum =a;
    ll div = b/a;
    if(a==b)
  {
        for(int i = 1 ; i<n ;i++){
            sum+=ans;
        }
    }
    else{

    for(int i = 1 ; i<n ; i++){

        ans  +=d ;
    sum+=ans;


       }
    }






    cout<<sum<<ee;


}

signed main() {
    fastin;
    ll tt=1 ;
   //cin>> tt;
    while(tt--) solve();


}