#include<bits/stdc++.h>
using namespace std;

#define ll long long
#define endl '\n'

long long toNum(string s){
    long long num = 0;
    for(int i = 0 ; i < s.size() ; i++){
        num = (num * 10) + (s[i] - '0');
    }
    return num;
}

void solve(){
    string s;
    ll n, a, b, ct = 0, d;
    cin >> s;
    cin.ignore();
    cin >> n;
    //cout << s << " " << n << endl;
    string tmp = "";
    for(int i = 0 ; i < s.size() ; i++){
        if(s[i] != '+') tmp += s[i];
        else if(ct == 0){
            //cout << "tmp -> " << tmp << endl;
            a = toNum(tmp);
            tmp = "";
            ct++;
        }else{
            b = toNum(tmp);
            break;
        }
    }
    d = b-a;
    //cout << a << " " <<  b << endl;
    cout << n * (2 * a + (n - 1) * d) / 2;
}

int main(){
    ios_base::sync_with_stdio(0), cin.tie(0);
    int t = 1;
    // cin >> t;
    while(t--){
        solve();
    }
    return 0;
}