//Как мне из
//listOf("a" to 2, "a" to 3)
//Сделать
//listOf("a" to 5)

fun main() {

    println(
        solve(
            listOf("a" to 2, "a" to 3) // -> [(a, 5)]
        )
    )

    println(
        solve(
            listOf("a" to 2, "a" to 3, "b" to 9, "c" to 0) // -> [(a, 5), (b, 9), (c, 0)]
        )
    )

    println(
        solve(
            listOf("a" to 2, "a" to 3, "b" to -1, "b" to 11, "c" to 0) // -> [(a, 5), (b, 10), (c, 0)]
        )
    )
}

fun solve(list: List<Pair<String, Int>>): List<Pair<String, Int>> {
    val result = mutableListOf<Pair<String, Int>>()
    val map = mutableMapOf<String, Int>()

    list.forEach {(k, v) ->
        map[k] = map.getOrDefault(k, 0) + v
    }

    val keys = map.keys
    keys.forEach {k ->
        result.add(k to map[k]!!)
    }

    return result
}