let inputCount = Int(readLine()!)!

var ans = 0
for _ in 0..<inputCount {
    let inpt = readLine()!.split(separator: " ").map({Int($0)!})
    ans += inpt[0] / 2 * inpt[1]
    if inpt[0] % 2 > 0 {
        ans += inpt[1]
    }
}

print(ans)