#include <iostream>
#include <algorithm>
#include <math.h>
using namespace std;

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