#include <bits/stdc++.h>

using namespace std;
ifstream fin ("grupuri.in");
ofstream fout("grupuri.out");
int k,n,a[100001];
void citire()
{fin>>k>>n;
 for(int i=1;i<=n;i++)fin>>a[i];
}
void rezolva ()
{int ct=0;
 sort(a+1,a+n+1,greater<int>());
 while(a[k]!=0)
    {if(a[k+1]==0)
        {ct+=a[k];
         break;
        }
     else
        {ct+=a[k]-a[k+1]+1;
         for(int i=1;i<=k;i++)
            a[i]-=a[k]-a[k+1]+1;

        }
    sort(a+1,a+n+1,greater<int>());
    }
 fout<<ct;
}
int main()
{       citire();
        rezolva();
        return 0;
}