#include<iostream>
#include<string>
#include<cmath>
#include<math.h>
//#include<array>
#include<algorithm>
#include<queue>
#include<deque>
#include<vector>
//#include<utility>
//#include<set>
#include<map>
//#include<list>
//#include<bitset>
//#include<iomanip>

using namespace std;
using LLI = long long int;
LLI N;

//配列,座標
/*array<int, 8>dir_x = { 0,0,1,1,1,-1,-1,-1 };
array<int, 8>dir_y = { 1,-1,1,0,-1,1,0,-1 };
array<int, 4>dir_x2 = { 0,0,1,-1 };
array<int, 4>dir_y2 = { 1,-1,0,0 };
char alp[] = { 'a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z' };*/

//Union Find
class UnionFind {
public:
	vector<int>data;
	UnionFind(int size) :data(size, -1) {};
	bool unionset(int x, int y) {
		x = root(x), y = root(y);
		if (x != y) {
			if (data[y] < data[x]) {
				swap(x, y);
			}
			data[x] += data[y];
			data[y] = x;
		}
		return x != y;
	}
	bool findset(int x, int y) {
		return root(x) == root(y);
	}
	int root(int x) {
		return data[x] < 0 ? x : data[x] = root(data[x]);
	}
	int size(int x) {
		return -data[root(x)];
	}
};

//最大公約数,最小公倍数
/*LLI gcd(LLI x, LLI y) {
	LLI  r;
	if (x < y) {
		swap(x, y);
	}
	r = x % y;
	while (r != 0) {
		x = y;
		y = r;
		r = x % y;
	}
	return y;
}
LLI lcm(LLI x, LLI y) {
	return x / gcd(x, y) * y;
}*/

//組み合わせ
/*long nCr(int n, int r) {
	long ans = 1;
	for (int i = n; i > n - r; i--) {
		ans = ans * i;
	}
	for (int i = 1; i < r + 1; i++) {
		ans = ans / i;
	}
	return ans;
}
long nHr(int n, int r) {
	n = n + r - 1;
	return nCr(n, r);
}*/

//素数判定
/*bool IsPrime(int num)
{
	if (num < 2) return false;
	else if (num == 2) return true;
	else if (num % 2 == 0) return false;

	double sqrtNum = sqrt(num);
	for (int i = 3; i <= sqrtNum; i += 2)
	{
		if (num % i == 0)
		{
			// 素数ではない
			return false;
		}
	}

	// 素数である
	return true;
}*/

//桁数
/*LLI GetDigit(LLI num) {
	LLI digit = 0;
	while (num != 0) {
		num /= 10;
		digit++;
	}
	return digit;
}*/

//サンプル
/*const int n_max = 8;
array<array<bool, n_max>, n_max>graph;
int dfs(int v,int N,array<bool,n_max>visited) {
	int ret = 0;
	bool all_visited = true;
	for (int i = 0; i < N;i++) {
		if (visited[i] == false) {
			all_visited = false;
		}
	}
	if (all_visited) {
		return 1;
	}
	for (int i = 0; i < N;i++) {
		if (graph[v][i] == false) {
			continue;
		}
		if (visited[i] == true) {
			continue;
		}
		visited[i] = true;
		ret += dfs(i,N,visited);
		visited[i] = false;
	}
	return ret;
}*/

//サンプル
/*vector<vector<int>>sub;
int dfs(int id) {
	if (sub[id].size() == 0) {
		return 1;
	}
	int minP = (int)1e9;
	int maxP = 0;
	for (int i : sub[id]) {
		int P = dfs(i);
		minP = min(minP, P);
		maxP = max(maxP, P);
	}
	return minP + maxP + 1;
}*/

//#define debug(x) cerr << #x << " : " << x << endl
//#define debugv(x) cerr << #x << " : "; for(auto i:x) cerr << i << ", "; cerr<<endl
//cout << "Yes" << endl;
//cout << "No" << endl;



int main(void) {
	LLI N,M,res = 0;
	cin >> N >> M;
	for (int i = 0; i < M;i++) {
		LLI a;
		cin >> a;
		res += a;
	}
	if (res > N) {
		cout << -1 << endl;
	}
	else {
		cout << N - res << endl;
	}
}