#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long double ld;
const ll mod = 1e9 + 7;
#define pival 3.14159265359
#define pll pair<ll,ll>
#define pb push_back
#define mp make_pair
#define fi first
#define se second
#define pqq priority_queue
#define all(a) a.begin(), a.end()
#define sz(a) (ll)(a.size())
#define endl '\n'
ll power(ll x, ll y, ll p)
{ 
    ll res = 1;  
    x = x % p;
    while (y > 0) { 
        if (y & 1) res = (res * x) % p; 
        y = y >> 1;
        x = (x * x) % p;   
    } 
    return res; 
}
int main()
{
	ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    cout.tie(NULL);
 	ll n, m;
 	cin >> n >> m;
 	ll sum = 0;
 	ll x;
	for (ll i = 0; i < m; i++) {
		cin >> x;
		sum += x;
	}   
	if (sum > n) {
		cout << -1 << endl;
	}
	else {
		cout << n - sum << endl;
	}
	return 0;
}