#include <iostream>
#include <string>
#include <vector>
#include <queue>
#include <stack>
#include <map>
#include <algorithm>
#include <numeric>
#include <functional>
#include <set>
#include <sstream>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <cctype>
#include <climits>

#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n) FOR(i,0,n)
#define FOE(i,a) for(auto i : a)
#define ALL(c) (c).begin(), (c).end()
#define EXIST(v, n) (std::find(v.begin(), v.end(), 1) != v.end())

using namespace std;


// intの各桁を配列に格納する
vector<int> int_to_vector(int n) {
    vector<int> v;
    while (n > 0) {
        v.push_back(n % 10);
        n /= 10;
    }
    reverse(v.begin(), v.end());
    return v;
}

int main(int argc, char *argv[]) {
    int K, N, F, a, s = 0;
    cin >> K >> N >> F;
    REP(i, F) {
        cin >> a;
        s += a;
    }
    cout << max(-1, K * N - s) << endl;
}