#include <bits/stdc++.h>

using namespace std;

int main()
{
    int a, b, c;
    cin >> a >> b >> c;
    vector<int> v(c, 0);
    
    for (int i = 0; i < c; ++i)
    {
        cin >> v[i];
    }
    
    int sum = accumulate(v.begin(), v.end(), 0);
    
    if (a*b >= sum)
        cout << (a*b) - sum << endl;
    else
        cout << -1 << endl;
}