#include <stdio.h>
#include <stdlib.h>

void solve(int n){
    if(n<0){
        printf("Invalid input");
        return;
    }
    if(n === 0){
        printf("0.0");
        return;
    }
    
    int last=n%10;
    int first = n;
    while(n>=10){
        first/=10;
    }
    double median = (double)(first+last)/2.0;
    printf("%.1f\n",median);
}
int main(){
    int n;
    if(scanf("%d",&n)!=1){
        return 1;
    }
    solve(n);
    return 0;
}