#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef vector<int> vec;
typedef vector<string> vecstr;
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define NO printf("NO")
#define YES printf("YES")

int main() {
    int k, n, f;
    cin >> k>>n>>f;
    int sum = k*n;
    rep(i, f) {
        int num;
        cin >> num;
        sum -= num;
    }
    if (sum < 0) {
        sum = -1;
    }
    cout << sum << endl;
}