#include <cstdio>
#include <iostream>
#include <sstream>
#include <fstream>
#include <iomanip>
#include <algorithm>
#include <cmath>
#include <string>
#include <vector>
#include <list>
#include <queue>
#include <stack>
#include <set>
#include <map>
#include <bitset>
#include <numeric>
#include <limits>
#include <climits>
#include <cfloat>
#include <functional>
using namespace std;

int main()
{
    int k, n, f;
    cin >> k >> n >> f;

    int ret = k * n;
    while(--f >= 0){
        int a;
        cin >> a;
        ret -= a;
    }
    ret = max(ret, -1);
    cout << ret << endl;

    return 0;
}