#M. 【例6.2】捡石头

Type: RemoteJudge

1000ms

64MiB

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说明

幼儿园小朋友昨天捡到3块小石头，想再去捡一块小石头，让这 $4$ 块石头正好一共重 $20$ 斤，请问他应该去捡一块多少斤的石头？

三个整数 $a$ $b$ $c$ ，是这三块石头的重量(斤)。( $0 < a,b,c,a+b+c≤20$ )

一个整数，是第四块石头的重量(斤)。

3 5 7