#include<iostream>
#include<math.h>
#include<algorithm>
#include<stdint.h>
#include<vector>
#include<deque>
#include<stack>
#include<string>
#include<cstring>
#include<time.h>
#include<iomanip>
#include<list>
#include<set>
#include<unordered_map>
#include<unordered_set>
#include<random>
#include<bitset>
#include <queue>
#include <map>


using namespace std;
using ll = long long;
using ull = unsigned long long;
using uint = unsigned int;
using ldouble = long double;

#define repi(i,a,b) for(ll i = (a); i < (b); i++)
#define rep(i, n) for(ll i = 0; i < (n); ++i)

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

