#include<bits/stdc++.h>
#define pb push_back
//#define int long long
#define mod 1000000007
#define mp make_pair
#define fi first
#define se second
#define ll long long
#define scan(n) ll a[n]; for(ll i = 0 ; i<n ; i++) cin>>a[i]
#define print(a,n) for(int i = 0 ; i < n ; i++) cout<<a[i]<<' '
#define fast ios_base::sync_with_stdio(false); cin.tie(0);cout.tie(0);
#define rep(i,x,y) for(ll i=x;i<y;i++)
using namespace std;
typedef vector<int>vi ;
typedef vector<ll>vll ;
typedef vector<vector<ll> >vvl ;
typedef pair<ll,ll> pll ;
/*vi adj[200005];
ll vis[200005];
ll parent[200005];
ll level[200005];*/
int main()
{
	fast;
	ll n,m;
	cin>>n>>m;
	scan(m);
	ll sum=0;
	for(ll i=0;i<m;i++)
	{
		sum+=a[i];
	}
	if(sum<=n)
	{
		cout<<n-sum;
	}
	else
		cout<<"-1";
	return 0;
}
