#include <iostream>
using namespace std;

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

    cin >> k >> n >> f;
    
    int a,sum;
    sum = 0;

    for(int i=0; i<f; i++){
        cin >> a;
        sum += a;
    }

    int kn = k * n;

    if(kn < sum) cout << -1 << endl;
    else cout << kn - sum << endl;
}