Editorial

If the number is negative, we will change its sign, making it positive - this will not change the first digit. We divide the number by 10 until there is only one digit left - this will be the first digit of the original number.

Implementation of the algorithm

We read an integer n.

scanf("%lld", &n);

If the number is negative, then we make it positive.

if (n < 0) n = -n;

We divide the number by 10 until it contains only the first digit.

while(n > 9)
    n /= 10;

We print the first digit of the number.

printf("%lld\n", n);

Java implementation

import java.util.*;

public class Main
{
    public static void main(String[] args)
    {
        Scanner con = new Scanner(System.in);
        long n = con.nextLong();
        if (n < 0) n = -n;

        while(n > 9)
            n /= 10;

        System.out.println(n);
        con.close();
    }
}