diff --git a/07/src/main.zig b/07/src/main.zig
index ea6314f..5d18297 100644
--- a/07/src/main.zig
+++ b/07/src/main.zig
@@ -80,9 +80,6 @@ fn run2(s: []const u8) !u64 {
         };
     }
 
-    var str_buf: [64]u8 = undefined;
-    var str_buf_fbs = std.io.fixedBufferStream(&str_buf);
-    var str_buf_writer = str_buf_fbs.writer();
     for (entries.items) |entry| {
         const nums: []u64 = entry.nums.items;
         std.debug.print("{}:", .{entry.n});
@@ -101,12 +98,11 @@ fn run2(s: []const u8) !u64 {
                     OP.ADD => v += n,
                     OP.MUL => v *= n,
                     OP.CAT => {
-                        try str_buf_fbs.seekTo(0);
-                        try std.fmt.formatInt(v, 10, std.fmt.Case.lower, .{}, &str_buf_writer);
-                        try std.fmt.formatInt(n, 10, std.fmt.Case.lower, .{}, &str_buf_writer);
-                        v = try std.fmt.parseInt(u64, str_buf[0..str_buf_fbs.pos], 10);
+                        const l = std.math.log10(n)+1;
+                        v = (v * std.math.pow(u64, 10, l)) + n;
                     },
                 }
+                if (v > entry.n) break;
                 op_i += 1;
             }