#include <iostream>
#include <utility>
#include <vector>
#include <queue>
#include <stack>
#include <array>
#include <algorithm>
#include <numeric>
#include <limits>
#include <string>
#include <regex>
#include <cstdio>
#include <cstring>
#include <cmath>
using namespace std;
using ll = long long;

int main()
{
    int n, m;
    cin >> n >> m;
    for (int i = 0; i < m; i++) {
        int a;
        cin >> a;
        n -= a;
    }
    if (n < 0) cout << -1 << endl;
    else cout << n << endl;
    return 0;
}
