#include <iostream>
using namespace std;
int main(void){
    
    
    int n,k,q;
    int sum = 0;
    cin >> n >> k >> q;
    n = n * k;
    
    for(int i = 0; i < q; i++) {
        int a;
        cin >> a;
        sum += a;
    }
    
    if(n > sum)cout << n - sum;
    else cout << -1;
}