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

int main(){
	int n, m;
	scanf("%d%d", &n, &m);
	int sum = 0;
	rep(i, m){
		int a;
		scanf("%d", &a);
		sum += a;
	}
	if(sum > n) cout << "-1" << endl;
	else cout << n - sum << endl; 
	return 0;
}