#include<stdio.h>
void find_median(int n){
    if(n<0){
        printf("Invalid input\n");
        return;
    }
    int first_digit=n;
    while(first_digit>=10){
        first_digit/-10;
    }
    int last_digit=n%10;
    float median=(first_digit=last_digit)/2.0;
    printf("%.1f",median);
}
int main(){
    int n;
    scanf("%d",&n);
    find_median(n);
    return 0;
}