#include <bits/stdc++.h>
using namespace std;
int main() {
ios::sync_with_stdio(false), cin.tie(0);
int n;
cin >> n;
long long ans = 0;
vector<int> a(n + 1);
for (int i = 1; i <= n; i++) {
cin >> a[i];
}
int l = 2, r = n - 1;
while (l <= n && a[l] > a[l - 1]) l++;
while (r >= 1 && a[r] > a[r + 1]) r--;
if (l > n || r < 1) {
cout << "0\n";
return 0;
}
while (l <= r) {
int add = min(abs(a[l] - a[l - 1]), abs(a[r] - a[r + 1])) + 1;
a[l] += add;
a[r] += add;
ans += add;
while (l <= n && a[l] > a[l - 1]) l++;
while (r >= 1 && a[r] > a[r + 1]) r--;
}
if (l <= n && r >= 1 && a[l] == a[r]) ans++;
cout << ans << endl;
return 0;
}
