#include <iostream>
#include <algorithm>
#include <utility>
#include <vector>
#include <string>
#include <map>
#include <queue>

using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
#define REP(i, n) for (int i = 0; i < (n); i++)
#define RREP(i, n) for (int i = (n) - 1; i >= 0; i--)
#define FOR(i, a, b) for (int i = (a); i < (b); i++)
#define RFOR(i, a, b) for (int i = (a); i > (b); i--)
#define ALL(a) (a).begin(), (a).end()

int main() {
    int N, M, sum = 0;
    cin >> N >> M;
    REP(i, M) {
        int A;
        cin >> A;
        sum += A;
    }
    if (N < sum) cout << -1 << endl;
    else cout << N - sum << endl;
    return 0;
}