#include<bits/stdc++.h>
using namespace std;
#define rep(i, n) for(int i=0; i<n; i++)
#define rep1(i, n) for(int i=1; i<=n; i++)
#define repr(i, n) for(int i=n-1; i>=0; i--)
#define repr1(i, n) for(int i=n; i>=1; i--)
#define all(v) v.begin(),v.end()
using ll = long long;
using pii = pair<int, int>;
using pil = pair<int, ll>;
using pli = pair<ll, int>;
using pll = pair<ll, ll>;
const int INF = 1e9;
const ll LLINF = 1e18;
const ll MOD = 1e9+7;
const double EPS = 1e-10;
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }

int main() {
    int n, m;
    cin >> n >> m;
    vector<ll> a(m);
    rep(i, m) cin >> a[i];

    ll sum = 0;
    rep(i, m) sum += a[i];
    ll ans = n - sum;
    if (ans >= 0) {
        cout << ans << endl;
    } else {
        cout << -1 << endl;
    }
}
