#include<bits/stdc++.h>
using namespace std;

int main()
{


        long long n,m,a,b,c,d,e,f,g,p,q,r,fr,sc,tr,sz,tz,sz1,sz2,i,j,k,mx=-9,mn=999999999999; string str,ttr,ntr;
        long long x=0,y=-1,cnt=0,res=0,ttl=0,ans=0,sum=0; long long flg=0,flag=1,na=0,as=1;
        vector<long long>u,v,w; vector< pair<long long,long long> >vct,tct; vector<string>vst;
        set<long long>st,nt,tt; map<long long,long long>mp,nq,qr,mp1,mp2;

        scanf("%lld %lld",&m,&n);

        for(i=1;i<=n;i++)
        {
            scanf("%lld",&a);  x=x+a;
        }


        printf("%lld\n",max(y,m-x));











    return 0;
}



