B.cpp

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<int, int> ii;
typedef vector<ii> vii;
typedef vector<int> vi;
typedef vector<ll> vll;
#define rep(i, a, n) for (int i = a; i < n; i++)
#define per(i, a, n) for (int i = n - 1; i >= a; i--)
#define pb push_back
#define eb emplace_back
#define fi first
#define se second
#define sz(x) ((int)(x).size())
#define all(a) (a).begin(), (a).end()
#define rall(a) (a).rbegin(), (a).rend()
#define INF 1000000000
#define MOD 1000000007

void solve() {
    int n,m;
    cin >> n >> m;
    ll s = 0;
    rep(i,0,m) {
        int tmp;
        cin >> tmp;
        s += tmp;
    }
    ll ans = n - s;
    if (ans < 0) ans = -1;
    cout << ans;
}

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    // freopen("input.txt", "r", stdin);
    // freopen("output.txt", "w", stdout);
    int t = 1;
    // cin >> t;
    while (t--)
        solve();
    return 0;
}