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