#include<bits/stdc++.h>
using namespace std;
long long m(long long a,long long b,long long n);//计算末项
int main(){
    long long a,b,n;
    cin>>a>>b>>n;
    cout<<(a+m(a,b,n))*n/2;//首项加末项之和乘项数除以2.
    return 0;
}
long long m(long long a,long long b,long long n){
    int c,d;
	d=b-a;
	c=a+(n-1)*d;
	return c;
}