C++ :

#include <iostream>
#include <cstdio>
#include <cstring>
using namespace std;
const int N = 10010;
char ch[N];
int main()
{
    string s;
    getline(cin , s);

    for (int i = 0; i < s.size(); ++i) {
        if(s[i] == 'A') cout << "T";
        if(s[i] == 'T') cout << "A";
        if(s[i] == 'G') cout << "C";
        if(s[i] == 'C') cout << "G";
    }

    return 0;
}