#include <algorithm>
#include <bitset>
#include <cassert>
#include <cctype>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>
#include <iomanip>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <sstream>
#include <string>
#include <vector>

using namespace std;

const int MAXN = 100;

int n, a[MAXN];

int main() {
    ios_base::sync_with_stdio(0);
    #ifdef ACMTUYO
        freopen("in.txt","r",stdin);
        freopen("out.txt","w",stdout);
        clock_t start = clock();
    #endif

    while (cin >> n) {
        int sum = 0;
        for (int i = 0; i < n; ++i) {
            cin >> a[i];
            sum += a[i];
        }
        int ans = 0;
        while (true) {
            int mag = -1, id = -1;
            for (int i = 1; i < n; ++i) {
                if (a[i] > mag) {
                    mag = a[i];
                    id = i;
                }
            }
            if (a[0] > mag)
                break;
            --a[id];
            ++a[0];
            ++ans;
        }
        cout << ans << '\n';
    }

    #ifdef ACMTUYO
        fprintf(stderr, "\ntime=%.3lfsec\n", 1. * (clock() - start) / CLOCKS_PER_SEC);
    #endif
    return 0;
}