#include <bits/stdc++.h>
#include <iostream>
//#include<>
using namespace std;

int main(void){
    int i,k,n,f,sum,a[100];
    cin>>k>>n>>f;
    for(i=0;i<f;i++){
        cin>>a[i];
        sum+=a[i];
    }
    if(sum<=k*n)
        return k*n-sum;
    else
    {
        return -1;
    }
}