a.cpp

#include <iostream>
#include <cmath>
#include <cstdio>
#include <algorithm>

using namespace std;

bool compar(int a, int b){
    return a > b;
}

int main(){

    int n, a[200], i, cnt = 0, j;
    cin >> n;
    for (i = 0; i < n; i++)
        cin >> a[i];
    sort(a + 1, a + n, compar);
    while (a[0] <= a[1]){
        a[0]++;
        a[1]--;
        j = 1;
        cnt++;
        while (j < n - 1 && a[j] < a[j + 1]){
            swap(a[j], a[j + 1]);
            j++;
        }
    }
    cout << cnt << endl;

    return 0;
}