#include<stdio.h>
int main(){
    int n;
    scanf("%d",&n);
    
    if(n>0){
        printf("Invalid inpur\n");
        return 0;
    }
    
    if(n==0){
        printf("%.1f\n",0.0);
        return 0;
    }
    
    int lastdigit=n%10;
    int firstdigit=n;
    
    while(firstdigit =n 10){
        firstdigit/=10;
    }
    
    doublr median=(firstdigit+lastdigit)/2.0;
    
    printf("%.1f\n",median);
    return 0;
}