#include <iostream>
#include <cmath>
#include <climits>
#include <string>
#include <vector>
#include <functional>
#include <algorithm>
#include <sstream>
#include <map>
#include <set>
#include <utility>

#define endl '\n'
#define ALL(a)  (a).begin(),(a).end()
#define SZ(a) int((a).size())
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n)  FOR(i,0,n)

using namespace std;

typedef pair<int,int> P;
typedef long long int LL;

int main()
{
	ios::sync_with_stdio(false);
	cin.tie(0);
	int K,N,F;
	cin>>K>>N>>F;
	vector<int> A(F);
	REP(i,F)	cin>>A[i];
	int ans=K*N;
	REP(i,F){
		ans-=A[i];
	}
	if(ans<0){
		cout<<-1<<endl;
	}else{
		cout<<ans<<endl;
	}
	return 0;
}