#include <cstdio>
#include <ctime>
#include <cstdlib>
#define Q 1000000
int c;
inline int nr(int x){
int a=0, q;
while(x!=0){
q=x/10;
a+=(x-(q<<1)-(q<<3)==c);
x=q;
}
return a;
}
int main(){
srand(time(0));
FILE *fin, *fout;
fin=fopen("cifre.in", "r");
fout=fopen("cifre.out", "w");
int a, b, k;
fscanf(fin, "%d%d%d%d", &a, &b, &c, &k);
double ans;
if(b-a+1<=Q){
int s=0;
for(int i=a; i<=b; i++)
s+=(nr(i)>=k);
ans=s/(double)(b-a+1);
}else{
int x, s=0;
for(int i=0; i<Q; i++){
x=a+rand()%(b-a+1);
if(nr(x)>=k) s++;
}
ans=s/(double)Q;
}
fprintf(fout, "%.4f\n", ans);
fclose(fin);
fclose(fout);
return 0;
}