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

int main(void){
    int i,k,n,f,sum;
    scanf("%d%d%d",&k,&n,&f);
    k*=n;
    for(i=0;i<f;i++){
       scanf("%d",&n);
       sum+=n;
    }
    if(sum<=k)
        return k*n-sum;
    else
    {
        return -1;
    }
}