#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define vi vector<ll>
#define vvi vector<vector<ll>>
#define vs vector<string>
#define vc vector<char>
#define vb vector<bool>
#define vp vector<pair<ll, ll>>
#define pp pair<ll, ll>
#define qi queue<ll>
#define qp queue<pp>
#define pqi priority_queue<ll>
#define pqp priority_queue<pp>
#define mi map<ll, ll>
#define mpi map<pp, ll>
#define mip map<ll, pp>
#define mpp map<pp, pp>
#define mb map<ll, bool>
#define si set<ll>
#define sp set<pp>
#define mod 1000000007
#define rep(a, b) for(int a = 0; a < (b); a++)
#define rep2(a, b) for(int a = 1; a < (b); a++)
#define inf 1000000000000000000
int main() {
ios_base::sync_with_stdio(false);
cin.tie(NULL);
int n;
cin >> n;
vi A(n);
rep(i, n) {
cin >> A[i];
}
int l = 0; int r = n - 1;
ll ans = 0;
while (l < r) {
while (l + 1 < n && A[l] < A[l + 1]) {
l++;
}
while (r-1 >= 0 && A[r] < A[r - 1]) {
r--;
}
if (l + 1 == r && A[l] == A[r]) {
ans++;
break;
}
if (!(l < r)) {
break;
}
int xl = A[l] - A[l + 1] + 1;
int xr = A[r] - A[r - 1] + 1;
int xmin = min(xl, xr);
A[l] -= xmin;
A[r] -= xmin;
ans += xmin;
}
cout << ans << endl;
}
