#include <bits/stdc++.h>
using namespace std;
#define ll long long int 
#define ull unsigned long long int
#define IOS ios_base::sync_with_stdio(false);cin.tie(NULL);
#define pb push_back
#define mp make_pair
#define fi first
#define se second
#define mx *max_element
#define mn *min_element
#define fo(i,n) for(ll i=0;i<n;i++)
#define of(i,n) for(ll i=n-1;i>=0;i--)
#define all(x) x.begin(),x.end()
#define sortall(x) sort(all(x))
#define mem(x,p) memset(x,p,sizeof(x))
#define w(t) ll t;cin>>t;while(t--)
#define kk "\n"
const ll mod=1e9+7;
int main()
{
  IOS
  ll n,m;
  cin>>n>>m;
  ll a[m],s=0;
  fo(i,m)
  {
    cin>>a[i];
    s=s+a[i];
  }
  ll r=-1;
  cout<<max(r,n-s)<<kk;
  return 0;
}