#include <bits/stdc++.h>
using namespace std;

//--------------------------------------------
#define FOR(i,a,b) for(int i=(a);i<(b);i++)
#define RFOR(i,a,b) for(int i=(b)-1;i>=(a);i--)
#define REP(i,n) FOR(i,0,n)
#define RREP(i,n) RFOR(i,0,n)
#define LL long long
#define INF INT_MAX/3

//--------------------------------------------
const double EPS = 1e-14;
const double PI  = acos(-1.0);

//--------------------------------------------

int main(){
  int n,k,f;
  int a[101];

  scanf("%d %d %d",&n,&k,&f);

  REP(i,f) scanf("%d",&a[i]);

  int sum = n * k;
  REP(i, f) sum -= a[i];

  if (sum >= 0) printf("%d\n",sum);
  else printf("-1\n");
}