#include<iostream>
#include<string>
#include<algorithm>
#include<cctype>
#include<set>
#include<bitset>
#include<math.h>
#include<map>
#include<queue>
#include<iomanip>
using namespace std;

int main(){
    int k, n, f, sum1 = 0;
    cin >> k >> n >> f;
    for (int i = 0; i < f; i++){
        int t;
        cin >> t;
        sum1 += t;
    }
    if (k*n >= sum1) cout << k*n - sum1 << endl;
    else cout << -1 << endl;
}