#include <iostream>
#include <cstring>
#include <vector>
#include <set>
#include <map>
#include <sstream>
#include <cstdio>
#include <algorithm>
#include <stack>
#include <queue>
#include <cmath>
#include <iomanip>
//#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const ll INF = (1 << 30);
const ll inf = (1LL << 60LL);
const ll maxn = 3e5 + 5;

int main()
{
    vector<int>vect;
    int n;
    cin>>n;
    int a[n + 50];
    for(int i=0;i<n;i++)
    {
        cin>>a[i];
    }
    int p=a[0];
    for(int i=1;i<n;i++)
    {
        if(a[i]>=p)
        {
            vect.push_back(a[i]);
        }
    }
    int k=0;

    if((int)vect.size() > 0){
    sort(vect.begin(),vect.end());

    int maks=vect[((int)vect.size()-1)];
    int j=(int)vect.size()-1;
    //cout<<"tuka"<<p<<endl;
    while(maks>=p)
    {
        vect[j]=vect[j]-1;
        ++p;
        sort(vect.begin(),vect.end());
        ++k;
        maks=vect[((int)vect.size()-1)];
    }
    }

    cout<<k<<endl;
}