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)

//AUTHOR:BlackBox Inc.//
#include<bits/stdc++.h>
#include<unordered_map>
#include<tr1/unordered_map>
#define ll long long
#define ull unsigned long long
#define vi vector<int>
#define vlli vector<long long int>
#define vvi vector<vector<int>>
#define vpii vector<pair<int,int>>
#define vs vector<string>
#define lin1(x) ll int x; x = scan();
#define lin2(x,y) ll int x, y; x = scan(), y = scan();
#define lin3(x,y,z) ll int x, y, z; x = scan(), y = scan(), z = scan();
#define pb(x) push_back(x)
#define makep(x,y) make_pair(x,y)
#define mx(a,b) (a)>(b)?(a):(b)
#define mn(a,b) (a)<(b)?(a):(b)
#define fori(s,e) for(i=s;i<=e;i++)
#define forj(s,e) for(j=s;j<=e;j++)
#define fork(s,e) for(k=s;k<=e;k++)
#define rep(i,s,e) for(int i=s;i<=e;i++)
#define brep(i,s,e) for(int i=s;i>=e;i--)
#define all(x) x.begin(),x.end()
#define mem(x,y) memset(x,y,sizeof(x));
#define bits1(x) __builtin_popcount(x)
#define pi 3.14159265358979323846264338327950
#define MOD7 1000000007
#define MOD9 1000000009
#define fast std::ios::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL)
using namespace std;
using namespace std::tr1;

int main()
{
#ifdef RAHUL
    freopen("input.txt","r",stdin);
#endif


ll a, b;
cin>>a>>b;

ll arr[b];
ll sum=0;
for (int i = 0; i < b; ++i)
{
	/* code */
	cin>>arr[i];
	sum+=arr[i];
}

if(a-sum<0){
	cout<<"-1"<<endl;
}
else{
	cout<<a-sum<<endl;
}

return 0;
}