#include <cassert>
#include <iostream>
#include <vector>
using namespace std;

int main() {
  int n;
  cin >> n;
  vector<int> lb(n);
  for (int &b: lb) cin >> b;

  int ans = 0;
  int cnt = 0;
  for (int i = 0; i < n-1; ++i) {
    cnt += lb[i];
    // cout << i << " " << lb[i] << " " << cnt << endl;
    if (cnt > i+1) {
      assert(lb[i] > 1);
      int diff = cnt-(i+1);
      lb[i+1] += diff;
      lb[i] -= diff;
      ans += diff;
      cnt -= diff;
    }
  }
  cnt = 0;
  for (int i = n-1; i > 0; --i) {
    cnt += lb[i];
    if (cnt > n-i) {
      assert(lb[i] > 1);
      int diff = cnt-(n-i);
      lb[i-1] += diff;
      lb[i] -= diff;
      assert(lb[i] == 1);
      ans += diff;
      cnt -= diff;
    }
  }
  cout << ans << endl;
}