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

int main()
{
    int n, k, f;
    cin >> n >> k >> f;
    
    vector<int> vec(f);
    for (int i = 0; i < f; ++i) cin >> vec[i];
    
    int ages = 0;
    for (int i = 0; i < f; ++i) ages += vec[i];
    
    int sum = n * k;
    int ans = sum - ages;
    
    if (ans >= 0) cout  << ans << "\n";
    else cout << -1 << "\n";
    
    return 0;
    
}