#include<bits/stdc++.h>
#define rep(i,n) for (int i = 0; i < n; i++)
using namespace std;
using ll = long long;

int main(){

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

