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

void main(){
    int n = readln.strip.to!int;
    long sum = 0;
    foreach(i;0..n){
        auto input = readln.strip.split.map!(to!int).array;
        sum += ceil(input[0] / 2f) * input[1];
    }
    writeln(sum);
}