#include<iostream>
using namespace std;
int main(){
    int k,n,f,mame;
    cin >> k >> n >> f;
    int a[f];
    mame = k*n;
    for(int i=0;i<f;i++){
        cin >> a[i];
        mame -= a[i];
    }
    if(mame<0)cout <<"-1\n";
    else cout << mame<<endl;
}