Calculating Pi with Thread

Hello I am trying to make a programm to calculate pi through Gottfried Leibniz formula (1-(1/3)+(1/5)-(1/7)+(1/9)...)=pi/4
I must make 4 threads. The first will calculate the formula for i=1 to 1000 the second 1000 to 2000 3rd 2000-3000 & 4th 3000-4000. I dont know how to use the count function connect each thread result and then sum it in order to calculate pi dividing by 4.
Here is the code I have made so far.

int main()
	{
	pthread_t t1,t2,t3,t4;
        (pthread_create(&t1,NULL,(void *)&count,"th1")
		{
		
		exit(1);
		}
	return 0;
	}
void count(void *str)
	{
		float denominator = 1.0;
		int sign = 1;
		float sum = 0;
		int i,j;

		for(j=i; j < i; j++) {
		sum += sign / denominator;
		sign = -sign;
		denominator += 2;
		}
	}

Thanks in Advance!
P.S. I want some hints not the solution. How to use count function and how to sum thread results.