#include <stdio.h>

double approximate_pi();

int main(void) {
    printf("pi is approximately equal to %f", approximate_pi());

    return 0;
}

double approximate_pi() {
    double pi = 1.0;
    int parity = 0;
    int i;

    for(i=3; i <= 99; i+=2) {
        if(parity == 0) {
            pi -= (1.0/i);
            parity = 1;
        }
        else if(parity == 1) {
            pi += (1.0/i);
            parity = 0;
        }
    }

    pi *= 4.0;

    return pi;
}