package chelper;

import io.InputReader;
import io.OutputWriter;
import misc.SimpleSavingChelperSolution;


public class H extends SimpleSavingChelperSolution {

  public void solve(int testNumber, InputReader in, OutputWriter out) {
    wrapSolve(testNumber, in, out);
  }

  @Override
  public void solve(int testNumber) {
    int n = in.nextInt();
    long[] a = in.nextLongArray(n);

    long[] needL = new long[n];
    long[] endL = new long[n];

    long[] needR = new long[n];
    long[] endR = new long[n];

    needL[0] = 0;
    endL[0] = a[0];

    for (int i = 1; i < n; i++) {
      endL[i] = Math.max(endL[i - 1] + 1, a[i]);
      needL[i] = needL[i - 1] + endL[i] - a[i];
    }

    needR[n - 1] = 0;
    endR[n - 1] = a[n - 1];

    for (int i = n - 2; i >= 0; i--) {
      endR[i] = Math.max(endR[i + 1] + 1, a[i]);
      needR[i] = needR[i + 1] + endR[i] - a[i];
    }

    long min = Long.MAX_VALUE;

    for (int i = 0; i < n; i++) {
      {
        long cost = needL[i];
        if (i < n - 1) {
          cost += needR[i + 1];

          if (endL[i] == endR[i + 1]) {
            cost++;
          }
        }
        min = Math.min(min, cost);
      }

      {
        long cost = needR[i];
        if (i > 0) {
          cost += needL[i - 1];

          if (endL[i - 1] == endR[i]) {
            cost++;
          }
        }
        min = Math.min(min, cost);
      }
    }

    out.println(min);
  }
}