#ifdef LOCAL
//#define _GLIBCXX_DEBUG
#endif
#include <bits/stdc++.h>
#include <random>
#ifndef LOCAL
//#pragma GCC optimize("O3")
//#pragma GCC optimize("Ofast")
//#pragma GCC optimize("unroll-loops")
#pragma GCC target("avx,avx2,bmi,bmi2,popcnt,lzcnt")
#endif
using namespace std;
using ll = long long;
using ld = long double;
#define x first
#define y second
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
mt19937_64 mt(418);
void solve();
void init();
int32_t main() {
#ifndef LOCAL
cin.tie(nullptr)->sync_with_stdio(false);
#endif
// srand(time(0));
init();
cout << fixed << setprecision(20);
int t = 1;
// cin >> t;
while (t--) {
solve();
}
}
void init() {}
#define int ll
const int N = 200200;
int a[N];
void solve() {
int n;
cin >> n;
for (int i = 0; i < n; i++) {
cin >> a[i];
}
int x = 0, y = n - 1;
ll ans = 0;
while (y > x && x < n - 1) {
while (y > x && x < n - 1 && a[x] < a[x + 1]) {
x++;
}
while (y > x && a[y] < a[y - 1]) {
y--;
}
if (y > x && x < n - 1) {
int c = min(a[x] - a[x + 1], a[y] - a[y - 1]) + 1, x1 = x, y1 = y;
ans += c;
if (a[x] - a[x + 1] < c) {
x++;
}
if (a[y] - a[y - 1] < c) {
y--;
}
a[x1] -= c;
a[y1] -= c;
}
}
cout << ans << '\n';
}
