diff --git a/dana/programs/calculator.dana b/dana/programs/calculator.dana
new file mode 100644
index 0000000..2ba3c6d
--- /dev/null
+++ b/dana/programs/calculator.dana
@@ -0,0 +1,119 @@
+def main
+    # Helper Function
+
+    def is_digit is byte: c as byte
+        if '0' <= c and c <= '9':   return: true
+        else:                       return: false
+
+    def is_operator is byte: c as byte
+        if c = '+' or c = '-' or c = '*' or c = '/':
+            return: true
+        else:
+            return false
+
+    (*
+        Conversion from part of a string to an int
+        [from, to)
+    *)
+    def string_to_int is int: s as byte[], from to as int
+        var i res pow is int
+
+        res := 0
+        pow := 1
+
+        i := to - 1
+        loop:
+            if i < from: break
+            res := res + pow * extend(s[i] - '0')
+            pow := pow * 10
+            i := i - 1
+
+        return: res
+
+    (*
+        Simple Calculator
+
+        Input: lines of the following form:
+        <Num1> <Op> <Num2>
+        where <NumX> is a positive integer
+        and <Op> is one of +, -, *, /
+        Any other characters among them get ignored.
+
+        Output: lines with the results
+        or a message "Invalid" in case of failure
+    *)
+
+    var buf is byte[256]
+    var idx len start end a b is int
+    var op is byte
+
+    loop lines:
+        readString: 256, buf
+
+        # Stop if we got nothing
+        len := strlen(buf)
+        if len = 0: break: lines
+
+        idx := 0
+
+        loop a_start:
+            if idx == len:
+                writeString: "Invalid\n"
+                continue: lines
+            elif is_number(buf[idx]):
+                break: a_start
+            idx := idx + 1
+        start := idx;
+
+        loop a_end:
+            if idx == len:
+                writeString: "Invalid\n"
+                continue: lines
+            elif not is_number(buf[idx]):
+                break: a_end
+            idx := idx + 1
+        end := idx;
+
+        a := string_to_int(buff, start, end);
+
+        loop op_detect:
+            if idx == len:
+                writeString: "Invalid\n"
+                continue: lines
+            elif is_operator(buf[idx]):
+                break: op_detect
+            idx := idx + 1
+        op := buf[idx]
+
+        loop b_start:
+            if idx == len:
+                writeString: "Invalid\n"
+                continue: lines
+            elif is_number(buf[idx]):
+                break: b_start
+            idx := idx + 1
+        start := idx;
+
+        loop b_end:
+            if idx == len or not is_number(buf[idx]):
+                break: b_end
+            idx := idx + 1
+        end := idx;
+
+        b := string_to_int(buff, start, end);
+
+        # We have finished parsing now we can calculate
+
+        if op = '+':
+            writeInteger: a + b
+        elif op = '-':
+            writeInteger: a - b
+        elif op = '*':
+            writeInteger: a * b
+        elif op = '/':
+            writeInteger: a / b
+        else:
+            writeString: "Invalid\n"
+
+    exit
+
diff --git a/dana/programs/calculator.input b/dana/programs/calculator.input
new file mode 100644
index 0000000..e6168cc
--- /dev/null
+++ b/dana/programs/calculator.input
@@ -0,0 +1,11 @@
+1+2
+4-3
+5*2
+100/4
+25 + 10
+  10 *   3  dsfdfadf
+asdf 5sdf -adfd2
+5-
+35+
++7
+2345 _ 2342
diff --git a/dana/programs/calculator.output b/dana/programs/calculator.output
new file mode 100644
index 0000000..5e662a4
--- /dev/null
+++ b/dana/programs/calculator.output
@@ -0,0 +1,11 @@
+3
+1
+10
+25
+35
+30
+3
+Invalid
+Invalid
+Invalid
+Invalid