#include <iostream>

#define p(s) cout << s << endl
#define c(s, _t, _f) if(c) _t; else _f;

using namespace std;

int main() {
    int k, n, f;
    cin >> k >> n >> f;
    
    int sum = k * n;
    
    int need = 0;
    for (int i = 0; i < f; i++) {
        int a = 0;
        cin >> a;
        need += a;
    }
    
    if (need <= sum) {
        p(sum - need);
    } else {
        p(-1);
    }
}