#include <cstdio>
#include <cstring>

int N, h[ 100001 ];
int Sol;

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