// #include <bits/stdc++.h>
#include <iostream>
#include <cmath>
#include <algorithm>
#include <map>
#include <unordered_map>
#include <vector>
#include <iomanip>
#include <string>
#include <queue>
#include <set>
#include <deque>
using namespace std;
#define int long long
#define endl "\n"
#define fi first
#define se second
const int M=998244353;
const int inf = 1e14;
const int LOG=17;
const int N=5e3+5;
int n , m , c , w , k , t=1 , q=1 , x , y , z , l , r;
void solve(){
cin >> n;
vector<int>a(n);
for (int i=0;i<n;i++){
cin >> a[i];
}
c=0;
int i=0,j=n-1;
while (j-i>1){
while (i<j and a[i]<a[i+1])i++;
while (j>i and a[j]<a[j-1])j--;
if (i==j)break;
int u=max(0ll,min(a[i]-a[i+1]+1,a[j]-a[j-1]+1));
a[i]-=u;
a[j]-=u;
c+=u;
while (i<j and a[i]<a[i+1])i++;
while (j>i and a[j]<a[j-1])j--;
}
if (i!=j and a[i]==a[j])c++;
cout << c << endl;
}
signed main()
{
// #ifndef ONLINE_JUDGE
// freopen("input.txt","r" ,stdin);
// freopen("output.txt","w",stdout);
// #endif
ios::sync_with_stdio(0);//DO NOT USE IN INTERACTIVE
cin.tie(0), cout.tie(0);//DO NOT USE IN INTERACTIVE
cout << fixed << setprecision(9);
srand(time(0));
// int t=1;
// cin >> t;
for (int _=1;_<=t;_++){
solve();
q++;
}
}
