C program for calculating pi using pthread doesn't give a correct approximation

Question

Below is some code I made that uses pthreads that calculates an approximation of pi.

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>

long num_terms;
double sum = 0.0;
pthread_mutex_t sum_lock;

void* calculate_pi(void* r) {
        long rank = (long) r;
        double factor;
        long n = num_terms / rank;
        long first = n * rank;
        long last = first + n;

        if (first % 2 == 0) {
                factor = 1.0;
        } else {
                factor = -1.0;
        }

        for (long i = first; i < last; i++, factor = -factor) {
                pthread_mutex_lock(&sum_lock);
                sum += factor / (2 * i + 1);
                pthread_mutex_unlock(&sum_lock);
        }

        return NULL;
}

int main(int argc, char* argv[]) {
        if (argc != 3) {
                printf("Error: must have exactly 3 arguments.\n");
                return 1;
        }

        long num_threads = strtol(argv[1], NULL, 10);
        num_terms = strtol(argv[2], NULL, 10);

        pthread_t* threads = malloc(num_threads * sizeof(pthread_t));
        pthread_mutex_init(&sum_lock, NULL);

        for (long i = 0; i < num_threads; i++) {
                pthread_create(&threads[i], NULL, calculate_pi, (void*) i + 1);
        }

        for (long i = 0; i < num_threads; i++) {
                pthread_join(threads[i], NULL);
        }

        sum = sum * 4.0;

        printf("Estimated value of pi: %f\n", sum);

        pthread_mutex_destroy(&sum_lock);
        free(threads);

        return 0;
}

Normally, when I run this program with the sample arguments 4 and 1000000, it's supposed to print out this:

Estimated value of pi: 3.141593

But instead, it prints out this, which isn't even close to the value of pi:

Estimated value of pi: -0.000001

Can you figure out what's wrong?