/*
  LANG: C++
  ID: Matteo Kinkela
  PROG: Razine
*/
#include <algorithm>

#include <cstdio>
#include <cstring>
#include <cctype>
#include <climits>
#include <cmath>

#include <vector>
#include <set>
#include <queue>
#include <stack>
#include <list>
#include <map>
#include <bitset>

#include <utility>
#include <valarray>

#include <string>
#include <iostream>

using namespace std;

const int MaxN = 100 + 10;

int N;
int Arr[ MaxN ];
int Niz[ MaxN ];
int Sol;

int main() {
    scanf( "%d", &N );
    for( int i = 0 ; i < N ; ++i ) {
         scanf( "%d", &Arr[ i ] );
         Niz[ i ] = Arr[ i ];
    } 
    for( int i = N - 2 ; i >= 0 ; --i ) {
         if( Arr[ i + 1 ] <= Arr[ i ] ) {
             Niz[ i ] -= ( Niz[ i ] - Niz[ i + 1 ] + 1 );
             //Sol += ( Niz[ i + 1 ] - Niz[ i ] );
         }
    }
    for( int i = 0 ; i < N ; ++i )
         Sol += abs( Niz[ i ] - Arr[ i ] );
    printf( "%d\n", Sol );
return 0;
}