// Correctness = 100%
// Performance = 100%

function solution(A){
    var storage = {};

    for( var i = 0; i < A.length; i++ ){
        var element = A[i];
        if( element in storage )
            storage[ element ] += 1;
        else
            storage[ element ] = 1;
    }

    for( var key in storage ){
        if( storage[ key ] % 2 === 1 )
            return +key;
    }
}