#include<bits/stdc++.h>

#define rep(i,n) for (int i =0;i<(n);i++)
#define LL long long 

using namespace std;

int main(){
    int a,b,x;
    int ans=0;
    cin >>a>>b>>x;
    int A=min(a,2*b),B=min(a,b);
    while(x>0){
        if(x-500<=0){
            ans+=B;
            x-=500;
        }
        else{
            ans+=A;
            x-=1000;
        }
    }
    cout <<ans<<endl;
    return 0;
}

