#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define pb push_back
#define rep(i,n) for(int i=0;i<(n);++i)

int main(){
	cin.tie(0);
	ios::sync_with_stdio(false);
	int k,n,f;cin >> k >> n >> f;
	int sum;
	for(int i=0;i<f;++i){
		int a;
		cin >> a;
		sum+=a;
	}
	if(k*n-sum>=0) cout << k*n-sum << endl;
	else cout << -1 << endl;
}