#include <sstream>
#include <string>
#include <vector>
#include <map>
#include <algorithm>
#include <iostream>
#include <utility>
#include <set>
#include <cctype>
#include <queue>
#include <stack>
#include <cstdio>
#include <cstdlib>
#include <cmath>

#define INF 1000000000

using namespace std;
typedef long long ll;

int main(void) {
    int K, N, F;
    cin >> K >> N >> F;
    int sum = K * N;
    for (int i = 0; i < F; i++) {
        int a;
        cin >> a;
        sum -= a;
    }
    if (sum < 0) cout << -1 << endl;
    else cout << sum << endl;
    return 0;
}