#include<iostream>
#include<string.h>
using namespace std;

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