#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <queue>
#include <cmath>
#include <climits>
#include <iomanip>
#include <set>
#include <map>
using namespace std;
typedef long long ll;
int main(){
    ll k,n,f;
    cin >> k >> f >> n;
    ll sum = 0;
    for(int i = 0;i < n;i++){
        ll in;
        cin >> in;
        sum += in;
    }
    cout << max((ll)-1,k * f - sum) << endl;
}