青蛙跳

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

typedef long long LL;

void solve(){
    LL a, b, k; cin >> a >> b >> k;
    cout << a * (k / 2 + (k % 2LL != 0)) - b * (k / 2) << endl;
}

int main(){
    int _; cin >> _;
    while(_ --) solve();
    return 0;
}