using namespace std;
typedef long long ll;
typedef unsigned long long ull;
#define _USE_MATH_DEFINES
#include <bits/stdc++.h>
#define int long long
#define rep(i,n) for(int i = 0; i < (n); i++)
#define divup(a,b) ( ( (a) + ( (b) - 1u) ) / (b) ) // a/b の余り切り上げ
// #include <boost/multiprecision/cpp_int.hpp>
// using namespace boost::multiprecision;
ll mod = 1000000007;
ll mod9 = 1000000009;

signed main(){
    ll n,m;
    cin>>n>>m;
    ll ans = n;
    rep(i,m){
        ll a;
        cin>>a;
        ans -= a;
    }
    if(ans < 0) ans = -1;
    cout<<ans<<endl;
}
