#include <bits/stdc++.h>
using namespace std;
int main(){
ios::sync_with_stdio(false);
cin.tie(nullptr);
int N;
cin >> N;
vector<long long> A(N), L(N), R(N);
// Read the initial heights A
for(int i = 0; i < N; i++){
cin >> A[i];
}
// 1) Build L: minimal strictly increasing (left -> right) with L[i] >= A[i].
// i.e. L[i] = max(A[i], L[i-1] + 1)
L[0] = A[0];
for(int i = 1; i < N; i++){
L[i] = max(A[i], L[i-1] + 1);
}
// 2) Build R: minimal strictly *decreasing* in left-to-right sense,
// which we can do by going right->left and enforcing R[i] >= R[i+1] + 1.
R[N-1] = A[N-1];
for(int i = N - 2; i >= 0; i--){
R[i] = max(A[i], R[i+1] + 1);
}
// 3) Final B: B[i] = min(L[i], R[i])
vector<long long> B(N);
for(int i = 0; i < N; i++){
B[i] = min(L[i], R[i]);
}
// 4) Compute the minimum # of water-intervals using the "difference array" trick
long long ans = 0LL;
long long prevX = 0LL; // x_0 = 0
for(int i = 0; i < N; i++){
long long x = B[i] - A[i]; // how many total increments needed at i
ans += max(0LL, x - prevX); // add positive jump
prevX = x;
}
// Output the result
cout << ans << "\n";
return 0;
}
