Error in decrypte caesar chipher

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1

Can anyone help me

public static String Decrypt(String str, int key, String letters){

	String decrypted= "";
	for(int pos=0;pos < str.length();++pos){
		char c=str.charAt(pos);
		if (c == ' '){
			decrypted += c;
			continue;
		}
		int ind=letters.indexOf(c);
		int ncpos=(ind-key)%26;
		decrypted += letters.charAt(ncpos);
	}
	return decrypted;
}

when run this error display
Exception in thread “main” java.lang.StringIndexOutOfBoundsException: String index out of range: 26
at java.lang.String.charAt(String.java:558)
at CaesarCipherLab.Decrypt(CaesarCipherLab.java:57)
at CaesarCipherLab.main(CaesarCipherLab.java:17)

2

What where the inputs to your method?