#include <bits/stdc++.h>
#define repp(i,x,n) for(int i=x;i<=n;i++)
using namespace std;
#define int long long
const int MAXN = 3e5+5;
const int mod = 1e9+7;
signed main () {
   int L,N;cin>>L>>N;
   int maks=0;
   repp(i,1,N){
    int x;cin>>x;
    int y;
    repp(j,1,x){
        cin>>y;
        maks=max(maks,y);
    }
   }
    cout<<max(0LL,maks*2-L)<<endl;
}
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