add approximation of pi

Makefile

@@ -0,0 +1,3 @@
+CFLAGS=-std=c99
+
+all: approximate-pi

approximate-pi.c

@@ -0,0 +1,23 @@
+#include <stdio.h>
+#include <stdbool.h>
+
+double approximate_pi(unsigned int steps) {
+  double approximate_pi = 4.0;
+  bool subtract = true;
+
+  for(int i=2; i<=steps; i++) {
+    if (subtract) {
+      approximate_pi -= (4.0/(i*2-1));
+    } else {
+      approximate_pi += (4.0/(i*2-1));
+    }
+    subtract = !subtract;
+  }
+
+  return approximate_pi;
+}
+
+int main(int argc, char *argv[]) {
+  // printf("sizeof(int) = %d\n", sizeof(int));
+  printf("pi = %8.7f\n", approximate_pi(10000000));
+}