#include <bits/stdc++.h>
// #include <ext/pb_ds/assoc_container.hpp> 
// #include <ext/pb_ds/tree_policy.hpp> 
// #include <ext/pb_ds/detail/standard_policies.hpp>
// using namespace __gnu_pdbs;
using namespace std;

// typedef tree<int, null_type, less<int>, rb_tree_tag,  
//             tree_order_statistics_node_update>  
//     ordered_set;

#define fi first
#define se second
#define mp make_pair
#define pb push_back
#define mt make_tuple
#define eb emplace_back
#define sf(x) sizeof(x)
#define len(x) x.length()
#define sz(x) x.size()
#define fastio() ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0)

#define LB(x,v) (lower_bound(x.begin(),x.end(),v)-x.begin())
#define UB(x,v) (upper_bound(x.begin(),x.end(),v)-x.begin())
typedef long long ll;
typedef pair<ll,ll> ii;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef unsigned long long ull;
typedef long double ld; 
typedef tuple<int,int,int> State;

ll mod = 1e9 + 7;

#define citc(tc) int tc; cin>>tc; while(tc--) 
#define each(a,x) for( auto &a:(x))
#define rep(i, n) for (ll i = 0; i < (n); i++)
#define repr(i, n) for (ll i = (n) - 1; i >= 0; i--)
#define rep2(i, l, r) for (ll i = (l); i < (r); i++)
#define rep2r(i, l, r) for (ll i = (r) - 1; i >= (l); i--)
#define range(a) a.begin(), a.end()
#define deb(x) cout<<#x<<" "<<x<<endl;
#define cini(i) int i; cin>>i;
#define cinll(l) ll l; cin>>l;
#define cinll2(l, z) ll l, z; cin>>l>>z;
#define cinll3(l, z, x) ll l, z, x; cin>>l>>z>>x;
#define cinai(a, n) vi a(n); rep(i, n) { cin>>a[i]; }
#define cinal(a, n) vl a(n); rep(i, n) { cin>>a[i]; }
#define coutai(a, n) rep(i, n) { cout<<a[i]<<" "; } ent;
#define coutal(a, n) rep(i, n) { cout<<a[i]<<" "; } ent;
#define ent cout<<endl;
#define ms(dp, x) memset(dp, x, sf(dp))

ll powm(ll a, ll b, ll mod) {
	ll res=1;
	while(b) {
		if(b&1)
			res=(res*a)%mod;
		a=(a*a)%mod;
		b>>=1;
	}
	return res;
}

void solve()
{
	ll n, m;
	cin>>n>>m;
	cinal(a, m);
	ll sum = 0;
	rep(i, m)
		sum += a[i];
	cout<<max(-1ll, n - sum)<<endl;
}
int main()
{
	fastio();
	// citc(tc)
		solve();
}
