#include <bits/stdc++.h>
#define REP(i, n) for(int i = 0; i < n; i++)
#define REP1(i,n) for(int i = 1; i < n+1 ;i++)
#define REPR(i, n) for(int i = n; i >= 0; i--)
#define FOR(i, m, n) for(int i = m; i < n; i++)
#define INF 2e9
#define ALL(v) v.begin(), v.end()
#define SZ(x) ((int)(x).size())
#define ARRAY_LENGTH(array) (sizeof(array) / sizeof(array[0]))
using namespace std;
typedef long long ll;
using p = pair<int, int>;
using P = pair<ll, ll>;
using V = vector<int>;
using VV = vector<vector<int>>;
using VVV = vector<vector<vector<int>>>;
using VL = vector<ll>;
using VVL = vector<vector<ll>>;
using VVVL = vector<vector<vector<ll>>>;
using Graph = VV;


int main(){
    ios::sync_with_stdio(false); cin.tie(0);
    int n,m;
    V a(m);ll sum =0;
    cin >> n >> m;
    REP(i,m){ cin >> a[i];
    sum+=a[i];
    }
    if(sum>n) cout << -1 << endl;
    else cout << n-sum << endl;
    


}
 