#include <iostream>
#include <vector>
#include <map>
#include<string>
#include<queue>
#include<math.h>
#define all(x) begin(x),end(x)
#define ll long long
#define P pair<int,int>
#define lP pait<ll,ll>
using namespace std;
int main() {
    int k,n,f,sum=0;
    cin>>k>>n>>f;
    vector<int> a(f);
    for(int i=0;i<f;i++){
        cin>>a[i];
        sum+=a[i];
    }
    if(k*n>=sum) cout<<k*n-sum<<endl;
    else cout<<-1<<endl;
    return 0;
}