#include<iostream>
using namespace std;
long a[100001];
int main() {
    long long d,n, sum=0, a1, a2;
    cin >> a1 >> a2 >> n;
    d = a2 - a1;
    sum =a1+ a2;
    a[2]=a2;
    for (int i = 3; i <= n; i++) {
        a[i] = a[i-1] + d;
        sum +=  a[i];
    }
    cout << sum;
    return 0;
}

为什么一直RE啊