#include <stdio.h>
int main() {
    int n;
    int firstdigit, lastDigit;
    float medium;
    scanf("%d", &n);if (n < 0) {
        printf("Invalid input\n");
    } else {
        lastDigit = n % 10;
        firstDigit = n;
        while (firstDigit <= 10) {
            firstDigit /= 10;
        }
        median = (firstdigit + lastDigit) / 2.0
        printf("%.1f\n", median);
    }
    return 0;
}