#define _USE_MATH_DEFINES // for C++  
#include <cmath> 
#include <iostream>
#include <iomanip>
#include <string>
#include <vector>
#include <algorithm>
#include <array>
#include <iterator>
#include <utility>
#include <set>
#include <random>
#include <map>
#include <stack>
#include <queue>
#include <list>
#include <functional>
#include <bitset>
#define LL long long

using namespace std;

int main() {
    int k, n, f;
    int s = 0;
    cin >> k >> n >> f;
    for (size_t i = 0; i < f; i++)
    {
        int tmp;
        cin >> tmp;
        s += tmp;
    }
    if (k * n < s )
    {
        cout << -1 << endl;
    }
    else
    {
        cout << k*n - s << endl;
    }
}