C++ :

#include<bits/stdc++.h>
using namespace std;
string a,b;
int main(){
	cin>>a;
	if(a=="red"){
		cout<<"stop！";
	}
	if(a=="yellow"){
		cout<<"stop！";
	}
	if(a=="green"){
		cout<<"go！";
	}
	return 0;
}

Python :

# coding=utf-8
x=input()
a='stop！'
b='go！'
if x=="red":
    print(a)
if x=="yellow":
    print(a)
if x=="green":
    print(b)