-9 + {-3 + [-7 + (-3-7) + (+9-2) + 1]}
-9 + {-3 + [-7 + (-10) + (+7) + 1]}
-9 + {-3 + [-7 - 10 + 7 + 1]}
-9 + {-3 + [-17 + 7 + 1]}
-9 + {-3 + [-10 + 1]}
-9 + {-3 + -9}
-9 + {-12}
-9 - 12
-21
