#include <stdio.h>

int main(int argc, const char **argv)
{
    int cardp1 = 0, card0 = 0, cardm1 = 0, choice = 0, max = 0;
    int loop1 = 0, loop2 = 0;

    fscanf(stdin, "%d %d %d %d", &cardp1, &card0, &cardm1, &choice);

    for ( loop1 = 0; loop1 < cardp1; loop1++ )
    {
        max++;
        choice--;
    }
    choice -= card0;
    if ( choice <= cardm1 )
    {
        for ( loop2 = 0; loop2 < choice; loop2++ )
        {
            max--;
            choice--;
        }
    }

    fprintf(stdout, "%d\n", max);
    return 0;
}