#include <cstdio>
#include <algorithm>

using namespace std;

int niz[ 105 ];

int main( void )
{
	int n;
	scanf( "%d", &n );
	
	for( int i = 0; i < n; ++i )
		scanf( "%d", &niz[ i ] );
		
	int zad = niz[ n - 1 ];
	
	int sol = 0;
	for( int i = n - 2; i >= 0; --i )
		if( niz[ i ] >= niz[ i + 1 ] )
		{
			sol += niz[ i ] - niz[ i + 1 ] + 1;
			niz[ i ] = niz[ i + 1 ] - 1;	
		}
	
	printf( "%d\n", sol );
	
	return 0;	
}