#include <bits/stdc++.h>
#define rep(i,a,b) for(int i = a; i < b; ++i)
#define all(c) c.begin(), c.end()
#define gmax(x,y) x=max(x,y)
#define gmin(x,y) x=min(x,y)
#define gadd(x,y) x=add(x,y)
#define gmul(x,y) x=mul(x,y)
using namespace std;

typedef pair<int,int> pii;
typedef long long ll;
typedef vector<int> vi;

int main(){
	cin.tie(0);
	cout.tie(0);
	ios_base::sync_with_stdio(0);
	int n,m;
	cin >> n >> m;
	vector<int> v(m);
	rep(i,0,m)cin >> v[i];
	ll ans = n - accumulate(all(v), 0LL);
	cout << (ans >= 0?ans:-1) << '\n';
}
