#include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for (int (i)=(0);(i)<(int)(n);++(i))
using ll = long long;
using P = pair<int, int>;
using namespace std;

int main() {
    int K, N, F;
    cin >> K >> N >> F;
    vector<int> A(F);
    int sum = 0;
    rep(i, F) {
        cin >> A[i];
        sum += A[i];
    }

    int ans = K*N - sum;
    cout << (ans < 0 ? -1 : ans) << endl;

    return 0;
}