Problem:
Score : 200 points

Problem Statement

Takahashi has N days of summer vacation.

His teacher gave him M summer assignments. It will take A_i days for him to do the i-th assignment.

He cannot do multiple assignments on the same day, or hang out on a day he does an assignment.

What is the maximum number of days Takahashi can hang out during the vacation if he finishes all the assignments during this vacation?

If Takahashi cannot finish all the assignments during the vacation, print -1 instead.

Constraints

1 \leq N \leq 10^6

1 \leq M \leq 10^4

1 \leq A_i \leq 10^4

Input

Input is given from Standard Input in the following format:

N M
A_1 ... A_M

Output

Print the maximum number of days Takahashi can hang out during the vacation, or -1.

Sample Input 1

41 2
5 6

Sample Output 1

30

For example, he can do the first assignment on the first 5 days, hang out on the next 30 days, and do the second assignment on the last 6 days of the vacation. In this way, he can safely spend 30 days hanging out.

Sample Input 2

10 2
5 6

Sample Output 2

-1

He cannot finish his assignments.

Sample Input 3

11 2
5 6

Sample Output 3

0

He can finish his assignments, but he will have no time to hang out.

Sample Input 4

314 15
9 26 5 35 8 9 79 3 23 8 46 2 6 43 3

Sample Output 4

9

Solution in C++ (GCC 9.2.1) (judge verdict: Accepted)

#include <algorithm>     //sort,二分探索,など
#include <bitset>        //固定長bit集合
#include <cmath>         //pow,logなど
#include <complex>       //複素数
#include <deque>         //両端アクセスのキュー
#include <functional>    //sortのgreater
#include <iomanip>       //setprecision(浮動小数点の出力の誤差)
#include <iostream>      //入出力
#include <iterator>      //集合演算(積集合,和集合,差集合など)
#include <map>           //map(辞書)
#include <numeric>       //iota(整数列の生成),gcdとlcm(c++17)
#include <queue>         //キュー
#include <set>           //集合
#include <stack>         //スタック
#include <string>        //文字列
#include <unordered_map> //イテレータあるけど順序保持しないmap
#include <unordered_set> //イテレータあるけど順序保持しないset
#include <utility>       //pair
#include <vector>        //可変長配列
// #include "boost/multi_array.hpp"

//名前空間
using namespace std;
typedef long long ll;

//マクロ
#define FOR(i, a, b) for (ll i = (a); i < (b); i++)  //range(a, b, 1)と対応 aからb回iter
#define FORD(i, a, b) for (ll i = (a); i > (b); i--) //range(a, b, -1)と対応 aからb回iter
#define REP(i, b) FOR(i, 0, b)
#define REPD(i, a) FOR(i, a, 0)
#define ALL(x) (x).begin(), (x).end() //sortなどの引数を省略したい
#define SIZE(x) ((ll)(x).size())      //sizeをsize_tからllに直しておく
#define MAX(x) *max_element(ALL(x))
#define INF 1000000000000 //10^12
#define MOD 10000007      //10^9+7
#define PB push_back
#define MP make_pair
#define F first
#define S second
#define MAXR 100000 //10^5:最大のrange(素数列挙などで使用)
#define CC(x) cout << (x) << endl

signed main()
{
    int N, M, a = 0, tmp = 0;
    cin >> N >> M;
    //cout << N << M << endl;
    REP(i, M)
    {
        cin >> tmp;
        a += tmp;
    }
    if (N - a < 0)
        cout << -1 << endl;
    else
        CC(N - a);
}