#include <bits/stdc++.h>
using namespace std;
int main(){
    int K,N,F,A,total,answer;
    cin >>K>>N>>F;
    total = 0;
    for(int i = 0;i<F;i++){
        cin>>A;
        total += A;
    }
    if((K*N)-total>=1){
        cout<<(K*N)-total;
    }
    else{
        cout<<-1;
    }
}