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diff --git a/challenge-143/paulo-custodio/python/ch-1.py b/challenge-143/paulo-custodio/python/ch-1.py
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+#!/usr/bin/python3
+
+# TASK #1 > Calculator
+# Submitted by: Mohammad S Anwar
+# You are given a string, $s, containing mathematical expression.
+# 
+# Write a script to print the result of the mathematical expression. To keep
+# it simple, please only accept + - * ().
+# 
+# Example 1:
+#     Input: $s = "10 + 20 - 5"
+#     Output: 25
+# Example 2:
+#     Input: $s = "(10 + 20 - 5) * 2"
+#     Output: 50
+
+import sys
+import re
+
+def expr(input):
+    input, value = factor(input)
+    while True:
+        input = input.strip()+" "
+        if input[0]=='*':
+            input = input[1:]
+            input, b = factor(input)
+            value *= b
+        elif input[0]=='/':
+            input = input[1:]
+            input, b = factor(input)
+            value /= b
+        else:
+            return input, value
+
+def factor(input):
+    input, value = term(input)
+    while True:
+        input = input.strip()+" "
+        if input[0]=='+':
+            input = input[1:]
+            input, b = term(input)
+            value += b
+        elif input[0]=='-':
+            input = input[1:]
+            input, b = term(input)
+            value -= b
+        else:
+            return input, value
+    
+def term(input):
+    input = input.strip()+" "
+    match = re.match(r"[-+]?\d+", input)
+    if match:
+        value = int(match.group(0))
+        input = input[match.end(0):]
+        return input, value
+    elif input[0]=='(':
+        input = input[1:]
+        input, value = expr(input)
+        input = input.strip()+" "
+        if input[0]!=')':
+            print("expected )")
+            sys.exit(1)
+        input = input[1:]
+        return input, value
+    else:
+        print("expected ( or number")
+        sys.exit(1)
+
+input = " ".join(sys.argv[1:])
+input, value = expr(input)
+print(value)