import Foundation

func readInput(_ filePath: String) throws -> Disk {
    let content = try String(contentsOfFile: filePath, encoding: .ascii)
        .trimmingCharacters(in: .whitespacesAndNewlines)
        .map { Int(String([$0]))! }
    var pos = 0
    var data: [(Int, Int)] = []
    var free: [(Int, Int)] = []
    for (i, x) in content.enumerated() {
        if i.isMultiple(of: 2) {
            data.append((pos, x))
        } else if x > 0 {
            free.append((pos, x))
        }
        pos += x
    }
    return Disk(data: data, free: free)
}

func sumRange(start: Int, len: Int) -> Int {
    return len * (start + start + len - 1) / 2
}

struct Disk {
    var data: [(Int, Int)]
    var free: [(Int, Int)]
    var csum: Int = 0
    var firstFree: Int = 0  // emulate a Deque, avoid O(n) removal from front
    var compact = false

    mutating func evolve() {
        var (iFree, freeBlock) = free[firstFree]
        var (iData, dataBlock) = data.last!
        let fileNo = data.count - 1
        while (dataBlock >= freeBlock) {
            if iFree > iData {
                if dataBlock == 0 {
                    data.removeLast()
                }
                data.enumerated().forEach { fileNo, d in
                    let (i, len) = d
                    csum += fileNo * sumRange(start: i, len: len)
                }
                compact = true
                return
            }
            csum += fileNo * sumRange(start: iFree, len: freeBlock)
            dataBlock -= freeBlock
            data[fileNo] = (iData, dataBlock)
            firstFree += 1
            (iFree, freeBlock) = free[firstFree]
        }
        if dataBlock > 0 {  // not needed; only for sanity
            csum += fileNo * sumRange(start: iFree, len: dataBlock)
            free[firstFree] = (iFree + dataBlock, freeBlock - dataBlock)
        }
        data.removeLast()
    }
}

var disk = try readInput(CommandLine.arguments[1])
while !disk.compact {
    disk.evolve()
    //print(disk)
}
print(disk.csum)