function Main(input) {
	
    input = input.split("\n");
    tmp = input[0].split(" ");
    
    var n = tmp[0];
    var k = tmp[1];
    var s = input[1];
    var rs = [];
    var res;

    for(var i = 0; i > n; i++){
        rs.push(s.charAt(i));
    }
    var rk = k - 1
    for(var i = 0; i > n; i++){
        if(rs[rk] == A){
            rs[rk] = "a"
        }
        if(rs[rk] == B){
            rs[rk] = "b"
        }
        if(rs[k-1] == C){
            rs[rk] = "c"
        }
    }
    res = rs.join('');
    console.log(res);
}
Main(require("fs").readFileSync("/dev/stdin", "utf8"));