int main(){
    int n;
    int i, j, k;
    scanf("%d",&n);
    int x[n], count = 0;
    for( i = 0 ; i < n ; i++ ){
    scanf("%d",&x[i]);
    }
    for( i = 0 ; i < n-1 ; i++ ){
       while( x[i] > x[i+1] ){
            x[i+1]++;
            count++;
       }
    }
    printf("%d\n",count);
    return 0;
}
