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 {
            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

    mutating func compact() {
        for fileNo in (0..<data.count).reversed() {
            var moved = false
            let (iData, dataBlock) = data[fileNo]
            for freeNo in 0..<free.count {
                let (iFree, freeBlock) = free[freeNo]
                if iFree > iData {
                    break
                }
                if freeBlock >= dataBlock {
                    csum += fileNo * sumRange(start: iFree, len: dataBlock)
                    free[freeNo] = (iFree + dataBlock, freeBlock - dataBlock)
                    data[fileNo] = (iData, 0)
                    moved = true
                    break
                }
            }
            if !moved {
                csum += fileNo * sumRange(start: iData, len:dataBlock)
            }
        }
    }
}

var disk = try readInput(CommandLine.arguments[1])
disk.compact()
print(disk.csum)