#include<iostream>
using namespace std;

int main () {
    int i,K,N,F,a[111];
    cin >> K >> N >> F;
    for(i = 0;i < F;i++) cin >> a[i];
    int sum = K * N;
    for(i = 0;i < F;i++) {
        sum -= a[i];
    }
     if(sum >= 0) cout << sum << endl;
     else cout << -1 << endl;
    return 0;
}