#include<iostream>
#include<vector>
using namespace std;
typedef long long ll;

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