#include<iostream>
#include<vector>
#include<algorithm>
#include<cstdio>
using namespace std;
int main(){
    int K,N,F,x,sum=0;
    cin >> K >> N >> F;
    while(F--){
        cin >> x;
        sum+=x;
    }
    if(N*K-sum>=0)  cout << N*K-sum;
    else    cout << "-1";
    return 0;
}