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

int main(){
    
    int k,n,f,num,sum=0;
    cin>>k>>n>>f;
    int v[f];
    num = k*n;
    for(int i=0; i<f; i++){
        cin>>v[i];
        sum+=v[i];
    }
    if(sum>num)
        cout<<"-1"<<endl;
    else
        cout<<num-sum<<endl;
        
    
    
    
}