#include <stdio.h>

int getSize() {
    FILE *in = fopen("task.in", "r");
    int size = 0;

    for ( char ch; fscanf(in, "%c", &ch) == 1; size++ );

    fclose(in);

    return size;
}

void arraySort(int array[], int size) {
    for ( int i = 1; i < size; i++ ) {
        for ( int j = 1; j < size; j++ ) {
            if ( array[j-1] > array[j] ) {
                int temp = array[j-1];

                array[j-1] = array[j];
                array[j] = temp;
            }
        }
    }
}

int main() {
    FILE *in = fopen("task.in", "r");
    FILE *out = fopen("task.out", "w");
    int size = getSize();
    int array1[size];
    int array2[size];
    int counter = 0;
    char ch;

    for ( int i = 0; i < size && fscanf(in, "%c", &ch) == 1; i++ ) {
        array1[i] = ch - 48;
        array2[i] = ch - 48;
    }

    arraySort(array1, size);

    for ( int i = 0; i < size; i++ ) {
        if ( array1[i] != array2[i] ) {
            counter += 1;
        }
    }
    fprintf(out, "%d\n", counter/2);

    fclose(in);
    fclose(out);

    return 0;
}