#include <stdio.h>

int main(){
    long int A, B, C, K;
    scanf("%ld%ld%ld%ld", &A, &B, &C, &K);
    if(K<=A){
        printf("%ld", K);
    }else if(K<=A+B){
        printf("%ld", A);
    }else{
        printf("%ld", 2 * A + B - K);
    }
    return 0;
}