#include<bits/stdc++.h>
#define ll long long
using namespace std;
int main(){
    string x;
    cin>>x;
    ll n;
    cin>>n;
    ll a= x[0]-48;
    ll b= x[2]-48;
    //cout << a << ' '<<b<<endl;
    ll diff = b-a;
    ll arr[n];
    arr[0]=a;
    arr[1]=b;
    for(int i=2;i<n;i++){
        arr[i]=arr[i-1]+diff;
    }
    ll sum=0;
    for(int i=0;i<n;i++){
        //cout<< arr[i]<<' ';
        sum+=arr[i];
    }
    cout <<sum<<endl;
}