function main(input) {
    //Enter your code here
    var data = input.split('\n');
    var firstLine = data[0].split(' ');
    var len = firstLine[0];
    //process.stdout.write('length:'+len);
    var toFind = firstLine[1];
    //process.stdout.write('toFind:'+toFind);
    //process.stdout.write('\n');
    var arr = data[1].split(' '); 
    //process.stdout.write(arr);
    for(var i=len-1;i>=0;i--) {
        if(arr[i] == toFind){
            process.stdout.write(i+1);
            return;
        }
    }
    process.stdout.write(-1);
}