#include<cstdio>
#include<cstring>

char S1[255], S2[255];
int freq1[256], freq2[256];

int main(){
    gets(S1);
    gets(S2);
    int l1 = strlen(S1);
    int l2 = strlen(S2);
    for(int i=0; i<l1; ++i){
        ++freq1[(unsigned char)S1[i]];
    }
    for(int i=0; i<l2; ++i){
        ++freq2[(unsigned char)S2[i]];
    }
    double S = 0;
    for(int i=0; i<256; ++i){
        S+=freq1[i]*freq2[i];
    }
    printf("%.6lf\n", S/(l1*l2));
    return 0;
}