Any Range Addition(using recursive function)

#include<stdio.h>
int sum=0;
int num1(int number_1,int number_2);
int num2(int number_1,int number_2);
int main()
{
    int first_number,last_number;
    scanf("%d%d",&first_number,&last_number);
    if(first_number<=last_number)
    {
     num1(first_number,last_number);
    }
    else
    {
    num2(first_number,last_number);

    }
    return 0;
}


int num1(int number_1,int number_2)
{
    if(number_1<=number_2)
    {
        if(number_1==number_2)
        {
        printf("%d=",number_1);
        }
        else
        {
            printf("%d+",number_1);
        }
        sum=sum+number_1;
        number_1++;
        num1(number_1,number_2);
    }
    else
    {
        printf("%d",sum);
    }


    return 0;
}


int num2(int number_1,int number_2)
{
    if(number_1>=number_2)
    {
        if(number_1==number_2)
        {
        printf("%d=",number_1);
        }
        else
        {
            printf("%d+",number_1);
        }
        sum=sum+number_1;
        number_1--;
        num2(number_1,number_2);
    }
    else
    {
        printf("%d",sum);
    }


    return 0;
}