#include <iostream>
using namespace std;
const int n = 10;
int x[n];
int main() {
char c;
int s = 0;
while((c=cin.get())!= EOF){
x[c-'0']++;//перевод символов в цифры
}
for(int i = 0; i < n; i++)
s+=x[i]/2;
cout << s << endl;
return 0;
}
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