Castromen,
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<script src="http://code.jquery.com/jquery-1.11.1.js"></script>
<meta charset="UTF-8">
<title>Document</title>
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<script>
var Amount_of_dogovor = "7550856854р";
var number = Amount_of_dogovor.substring(0, Amount_of_dogovor.length - 1);
var number2 = 1;
var number3 = 10;
var sum1 = number2 - (-number);
var sum2 = number3 - (-number);
alert("summa1 =" + sum1 + " summa2 =" + sum2);
</script>