diff --git a/src/leibniz.zig b/src/leibniz.zig
index 7a3fabe..4805b91 100644
--- a/src/leibniz.zig
+++ b/src/leibniz.zig
@@ -1,20 +1,22 @@
 const std = @import("std");
 
-pub fn main() anyerror!void {
+pub fn main() !void {
+    // like C -ffast-math
+    @setFloatMode(.Optimized);
+
     var file = try std.fs.cwd().openFile("rounds.txt", .{});
     defer file.close();
     var buffer: [1024]u8 = undefined;
-    var n = try file.read(buffer[0..buffer.len]);
+    var n = try file.readAll(buffer[0..buffer.len]);
     var rounds = try std.fmt.parseInt(i64, std.mem.trim(u8, buffer[0..n], "\n"), 10);
     rounds += 2;
 
     var i: usize = 2;
-    var x: f64 = 1.0;
     var pi: f64 = 1.0;
     while (i < rounds) : (i += 1) {
-        x = -x;
+        const x: f64 = -1.0 + 2.0 * @intToFloat(f64, i & 1);
         pi += (x / @intToFloat(f64, 2 * i - 1));
     }
     pi *= 4;
-    try std.io.getStdOut().writer().print("{}", .{pi});
+    try std.io.getStdOut().writer().print("{d:.16}", .{pi});
 }