#include <stdio.h>

int main() {
    int n;
    scanf("%d" , &n);
    
    // check for negative input
    if (n < 0) {
        printf("Invalid input");
        return 0;
    }
    int last = n % 10; // Get last digit
    int first = n;
    
    // Find first digit
    while (first >= 10) {
        first = first / 10;
    }
    // calculate median
    float median = (first + last) / 2.0;
    
    // print result formatted to one
    decimal place
        printf("%.1f", median);
        return 0;
}