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#include <iostream>
using namespace std;
int main(int argc, char const *argv[])
{
int n, x, c, v;
while(cin >> n && n)
{
x = 0;
for (int i = 0; i < n; ++i)
{
cin >> c >> v;
x += v/2;
}
cout << x/2 << endl;
}
return 0;
}