#include <iostream>
#include <iomanip>

#include <algorithm>
#include <math.h>

#include <string>
#include <list>
/////////
#define REP(i, x, n) for(int i = x; i < n; i++)
#define rep(i,n) REP(i,0,n)
#define P(p) cout<<(p)<<endl;
/////////
typedef long long LL;
typedef long double LD;
/////////
using namespace::std;
/////////

int main(void){
	cin.tie(0);
	ios::sync_with_stdio(false);
	cout << fixed;//
	//cout << setprecision(6);//

	//なんだろう、難易度２倍がデフォ？すごいなぁ
	int K,N,F;
	cin>>K>>N>>F;
	int A;//
	int sum=0;
	rep(i,F){
		cin>>A;
		sum += A;
	}
	int mame = K*N;//[1,10^3]*[1*10^3]=[1,10^6]
	if( sum > mame){
		P(-1);
	}else{
		P(mame-sum);
	}
	return 0;
}