#include <iostream>
#include <vector>
#include <algorithm>

#define REP(i, a, b) for (int i = int(a); i < int(b); i++)

using namespace std;

typedef long long int lli;

int main() {
    int K, N, F;
    cin >> K >> N >> F;
    int sum = 0;
    REP (i, 0, F) {
        int Ai;
        cin >> Ai;
        sum += Ai;
    }
    int ans = K * N - sum;
    cout << (ans < 0 ? -1 : ans) << endl;
    return 0;
}