#import<fstream>
#define L 10000
using namespace std;ifstream f("cifre.in");ofstream g("cifre.out");long long x,p,n,a,b,k,c,nr;int cifre(int n){int nr=0;for(;n;n/=10)if(n%10==c)nr++;return nr;}int main(){f>>a>>b>>c>>k;if(b-a>=L){for(;a%L;++a,p++){if(cifre(a)>=k)nr++;}for(x=0;b-a>=L;a+=L,p+=L){x=cifre(a/L);if(x>=k)nr+=6561;if(x>=k-1)nr+=2916;if(x>=k-2)nr+=487;if(x>=k-3)nr+=35;if(x>=k-4)nr++;}}for(;a<=b;++a,++p)if(cifre(a)>=k)nr++;g<<(double)nr/p;return 0;}