#include<bits/stdc++.h>
#define int long long
#define endl '\n'
using namespace std;
int cl,cr,n,r;
signed main()
{
	ios::sync_with_stdio(0);
	cin.tie(0);
	cout.tie(0);
	cin>>r>>n;
	cr=r;
	while(n--)
	{
		int d;
		cin>>d;
		cl=max(min(cl+d,r),0ll);
		cr=max(min(cr+d,r),0ll);
	}
	if(cl!=cr)
	{
		cl=-1;
	}
	cout<<cl;
	return 0;
}