import std.stdio,
       std.conv,
       std.string,
       std.range,
       std.math,
       std.algorithm;

void main()
{
    auto N = readln.strip.to!int;
    long[] C, D;
    foreach (_; 0 .. N)
    {
        auto input = readln.split.to!(long[]);
        C ~= input[0];
        D ~= input[1];
    }

    0UL.reduce!((a,b) => a+(C[b]+1)/2*D[b])(0.iota(N)).writeln;
}