diff --git a/calc_str_eval.py b/calc_str_eval.py
new file mode 100644
index 0000000..254a179
--- /dev/null
+++ b/calc_str_eval.py
@@ -0,0 +1,52 @@
+import sys
+from typing import NoReturn, Union
+
+
+def readline() -> str:
+    """
+    Returns a stripped input.
+    """
+    input_line: str = sys.stdin.readline()
+    return input_line.rstrip()
+
+
+def parse_multi_or_div(equation: str) -> Union[int, float]:
+    return int(equation)
+
+
+def parse_add_or_minus(equation: str) -> Union[int, float]:
+    plus_index = equation.find("+")
+    minus_index = equation.find("-")
+
+    if plus_index == -1 and minus_index == -1:
+        return parse_multi_or_div(equation)
+    elif plus_index == -1:
+        return parse_multi_or_div(equation[:minus_index]) - parse_add_or_minus(equation[minus_index + 1:])
+    elif minus_index == -1:
+        return parse_multi_or_div(equation[:plus_index]) + parse_add_or_minus(equation[plus_index + 1:])
+    else:
+        if -1 < plus_index and plus_index < minus_index :
+            return parse_multi_or_div(equation[:plus_index]) + parse_add_or_minus(equation[plus_index + 1:])
+        elif -1 < minus_index and minus_index < plus_index :
+            return parse_multi_or_div(equation[:minus_index]) - parse_add_or_minus(equation[minus_index + 1:])
+
+
+def main() -> NoReturn:
+    equation: str = ""
+    while True:
+        input_line: str = readline()
+
+        if input_line == "quit":
+            exit(0)
+        elif "=" not in input_line:
+            equation += input_line
+            continue
+        else:
+            equation_end_index = input_line.find("=")
+            equation += input_line[:equation_end_index]
+            print(parse_add_or_minus(equation))
+            equation = input_line[equation_end_index + 1:]
+
+
+if __name__ == "__main__":
+    main()