#include <bits/stdc++.h>

#define REP(i, n) for(int i = 0; i < n; i++)
#define REPR(i, n) for(int i = n - 1; i >= 0; i--)
#define FOR(i, m, n) for(int i = m; i <= n; i++)
#define FORR(i, m, n) for(int i = m; i >= n; i--)
#define SORT(v, n) sort(v, v+n);
#define VSORT(v) sort(v.begin(), v.end());
#define VSORTR(v) sort(v.rbegin(), v.rend());
#define ALL(v) (v).begin(),(v).end()
using namespace std;
using ll = long long;
using vll = vector<ll>;
using vvll = vector<vector<ll>>;
using P = pair<ll, ll>;
const ll mod = 1e9+7;


int main(){
	
    cin.tie(0);
    cout.tie(0);
    ios::sync_with_stdio(false);

    ll n,m;cin>>n>>m;
    vll a(n); REP(i,n) cin>>a[i];
    ll sum = accumulate(ALL(a), 0LL);
    if (n>=sum){
        cout<<n-sum<<endl;
        return 0;
    }
    else cout <<-1<<endl;
    return 0;
}