#include <stdio.h>

int main(void) {
    int n = 1000000;
    double pi = 0.0;

    for (int i = 0; i < n; i++) {
        double term = (i % 2 == 0 ? 1.0 : -1.0) / (2 * i + 1);
        pi += term;
    }

    pi *= 4.0;
    printf("pi ≈ %.10f\n", pi);
    return 0;
}