#include <iostream>
#include <cstdio>
#define FOR(i,a,b) for(int i=(a);i<(b);i++)
#define REP(i,n)  FOR(i,0,n)
using namespace std;

int main() {
	int K,N,F;
	int sum=0;
	cin>>K>>N>>F;
	int A[F];
	REP(i,F){
		cin>>A[i];
		sum +=A[i];
	}
	if(sum>K*N){
		cout<<"-1"<<endl;
	}else{
		cout<<K*N-sum<<endl;
	}
	return 0;
}