#include <iostream>
#include <fstream>
#define N 100005
using namespace std;
ifstream f("grupuri.in");
ofstream g("grupuri.out");
int v[N],poz[N],heap[N],n,k,l,mn,inf=-(1<<30),rez,ch,maxime[N];
string s;

int nextint()
{
    int num=0;
    while(s[ch]==' ') ++ch;
    while(s[ch]<='9' && s[ch]>='0')
    {
        num=num*10+s[ch]-'0';
        ++ch;
    }
    return num;
}

void down(int x)
{
    bool ok=true;
    while(ok)
    {
        ok=false;
        mn=inf;
        if(x*2<=l && mn<v[heap[x*2]]) mn=v[heap[x*2]], ch=x*2;
        if(x*2+1<=l && mn<v[heap[x*2+1]]) mn=v[heap[x*2+1]], ch=x*2+1;
        if(v[heap[x]]<mn)
        {
            swap(poz[heap[x]],poz[heap[ch]]);
            swap(heap[x],heap[ch]);
            ok=true;
            x=ch;
        }
    }
}

void up(int x)
{
    while(v[heap[x]]>v[heap[x/2]])
    {
        swap(poz[heap[x]],poz[heap[x/2]]);
        swap(heap[x],heap[x/2]);
        x/=2;
    }
}

int main()
{
    f>>k>>n; getline(f,s); getline(f,s); l=n;
    for(int i=1;i<=n;++i)
    {
        v[i]=nextint();
        heap[i]=n-i+1;
        poz[i]=n-i+1;
    }
    while(l>=k)
    {
        for(int i=1;i<=k;++i) ///scoate maximele
        {
            maxime[i]=heap[1]; v[heap[1]]--;
            heap[1]=heap[l]; poz[heap[1]]=1; --l;
            down(1);
        }
        for(int i=1;i<=k;++i)
        {
            if(v[maxime[i]])
            {
                up(++l);
            }
        }
        ++rez;
    }
    g<<rez;
    return 0;
}