import java.util.*;

public class estoque{
    public static void main (String[] args){
        Scanner s = new Scanner(System.in);
        int n = s.nextInt();

        int[] h = new int[n];
        int[] pref = new int[n];
        int[] suf = new int[n];

        for( int i = 0; i < n; i++ ){
            h[i] = s.nextInt();
            pref[i] = h[i]; 
            if( i > 0 ) pref[i] = Math.min(pref[i], pref[i - 1] + 1); 
        }

        long ans = 0; 
        for( int i = n - 1; i >= 0; i-- ){
            suf[i] = h[i]; 
            if( i + 1 < n ) suf[i] = Math.min(suf[i], suf[i + 1] + 1);
            ans += Math.min(pref[i], suf[i]);
        }

        System.out.println(ans);
    }
}