import std.stdio, std.conv, std.string, std.algorithm, std.array;

void main(){
    const string[] input = readln.split;
    const int[] arr = input.map!(to!int).array;
    foreach(idx, val; arr){
        if(idx+1 != val){
            writeln(idx+1);
            return;
        }
    }
    writeln(10);
}