#include <iostream>
#include <vector>
#include <string>
#include <cmath>
#include <iomanip>
#include <algorithm>
using namespace std;
using ll = long long;

int main() {
    int K, N, F;
    cin >> K >> N >> F;
    int n = K*N;
    for(int i=0; i<F; ++i) {
        int a;
        cin >> a;
        n -= a;
    }
    cout << max(-1, n) << endl;
}