C language to calculate the triangle area code

//Area formula s = (a+b+c) / 2& PI; & have spent Area = SQRT (s * (s - a) * (s - b) * (s - c));
//So let's figure out the area of a triangle
int check(double a);
int check2(double a, double b, double c);
#include <stdio.h>
#include <math.h>
int main(void)
{
    double area = 0;
    double s;
    double a,b,c;
    printf(" Please enter the length of the three sides of the triangle (a b c):");
    scanf("%lf %lf %lf", &a, &b, &c);
    if (check(a) == 1 && check(b) == 1 && check(c) ==1)
    {
        if (check2(a,b,c) == 0)
        {
            printf(" You did not enter a triangle . Please re-enter nr");
            return 0;
        }
        else if (check2(a,b,c) == 1)
        {
            s = (a+b+c) / 2;
            area = sqrt(s * (s - a) * (s - b) * (s - c));
            printf(" The area of the triangle is :%gnr",area);
        }
    }
    else
        printf(" Input is wrong , Please re-enter .nr");
    return 0;
}
 
//Determine whether the input data is valid
int check(double a)
{
    if (a > 0)
        return 1;
    else
        return 0;
}
//Determine if the triangle is valid
int check2(double a, double b, double c)
{
    if ( ((a+b)<=c) | ((a+c)<=b) | ((c+b)<=a) )
        return 0;
    if (abs(a-b)>=c | abs(a-c)>=a | abs(c-b)>=a)
        return 0;
    else
        return 1;
}

  On second thought,area is not garbage, so you don't have to write so many lines to declare variables.

A few changes

//Area formula s = (a+b+c) / 2& PI; & have spent Area = SQRT (s * (s - a) * (s - b) * (s - c));
//So let's figure out the area of a triangle
int check(double a);
int check2(double a, double b, double c);
#include <stdio.h>
#include <math.h>
int main(void)
{
    double area,s,a,b,c;
    printf(" Please enter the length of the three sides of the triangle (a b c):");
    scanf("%lf %lf %lf", &a, &b, &c);
    if (check(a) == 1 && check(b) == 1 && check(c) ==1)
    {
        if (check2(a,b,c) == 0)
        {
            printf(" You did not enter a triangle . Please re-enter nr");
            return 0;
        }
        else if (check2(a,b,c) == 1)
        {
            s = (a+b+c) / 2;
            area = sqrt(s * (s - a) * (s - b) * (s - c));
            printf(" The area of the triangle is :%gnr",area);
        }
    }
    else
        printf(" Input is wrong , Please re-enter .nr");
    return 0;
}
 
//Determine whether the input data is valid
int check(double a)
{
    if (a > 0)
        return 1;
    else
        return 0;
}
//Determine if the triangle is valid
int check2(double a, double b, double c)
{
    if ( ((a+b)<=c) | ((a+c)<=b) | ((c+b)<=a) )
        return 0;
    if (abs(a-b)>=c | abs(a-c)>=a | abs(c-b)>=a)
        return 0;
    else
        return 1;
}