#include<bits/stdc++.h>
using namespace std;
int main()
{
    string s;
    cin>>s;
    int x=s[0]-'0';
    int y=s[2]-'0';
    int sum=0;
    int d=y-x;
    int b;cin>>b;
    sum+=(2*x+(b-1)*d);
    sum=(sum*b)/2;
    cout<<sum<<endl;
}