#include <cstdio>
#include <algorithm>

using namespace std;

int n, data [ 10005 ], res = 0;

int main ( void ) {

    scanf ( "%d", &n );

    for ( int i = 1; i <= n; ++i )
        scanf ( "%d", &data [ i ] );

    for ( int i = n; i > 0; --i ) {
        if ( data [ i ] <= data [ i - 1 ] ) {
            res += data [ i - 1 ] - data [ i ] + 1;
            data [ i - 1 ] = data [ i ] - 1;
        }
    }

    printf ( "%d\n", res );

    return 0;
}